<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:717a5a4d-c9b1-4ccc-8dd0-90e0d0ce4776,g:103877a4-cc1b-4269-9413-738f12d53091-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.magnite.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="mgni-20220630.xsd#CondensedConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="extended" id="i6a9fad80f7ce4ec08042d6f7140aa65d_CondensedConsolidatedStatementsofStockholdersEquity">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_3f4cccbc-2de7-403e-80b1-a1a5af086aad" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_3f4cccbc-2de7-403e-80b1-a1a5af086aad" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_47bd2ab3-114d-43b6-bddd-41da5e702de3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_47bd2ab3-114d-43b6-bddd-41da5e702de3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_d5f83bc1-6dcd-44dd-ae9c-5d7b0494d2d8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_TreasuryStockCommonShares_d5f83bc1-6dcd-44dd-ae9c-5d7b0494d2d8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a027335f-5a1a-4b7d-b4bc-3a15696c7864" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockholdersEquity_a027335f-5a1a-4b7d-b4bc-3a15696c7864" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_4cf850d6-74f1-4ff5-b689-5b62abd4ca18" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_4cf850d6-74f1-4ff5-b689-5b62abd4ca18" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e4552655-22d7-46fb-afa7-f64b2fba77f3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_e4552655-22d7-46fb-afa7-f64b2fba77f3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_c4454960-dd9e-42b6-bb4a-99486c55541d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_c4454960-dd9e-42b6-bb4a-99486c55541d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_20d6befd-9ee8-4adf-8f48-5c887ed9f2a5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_20d6befd-9ee8-4adf-8f48-5c887ed9f2a5" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_19d9b241-4e2d-4e18-99cf-d1194c24352a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_19d9b241-4e2d-4e18-99cf-d1194c24352a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_1e173baf-dfc1-41f9-92c6-e366497ab44d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_1e173baf-dfc1-41f9-92c6-e366497ab44d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_c46d0ad5-2bb1-4652-baa6-f8ea2da05b86" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_c46d0ad5-2bb1-4652-baa6-f8ea2da05b86" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_d3c0af82-aa5b-4a5f-8e39-4ff3fbe94072" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_d3c0af82-aa5b-4a5f-8e39-4ff3fbe94072" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_d760f2c5-7da2-4f8e-b333-77601af07e2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_d760f2c5-7da2-4f8e-b333-77601af07e2e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_7612d402-c433-49c0-b00b-399f53fb93a4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_7612d402-c433-49c0-b00b-399f53fb93a4" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_c3b12d90-f44e-401c-b512-08729dd7873c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_c3b12d90-f44e-401c-b512-08729dd7873c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_adec5320-e533-413e-b579-b3856dbc72ce" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_adec5320-e533-413e-b579-b3856dbc72ce" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9b2e9b97-3b3c-438b-ab3e-ae115442cb4e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9b2e9b97-3b3c-438b-ab3e-ae115442cb4e" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_1aed1272-0808-411a-bef9-294e7e485c08" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_1aed1272-0808-411a-bef9-294e7e485c08" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_APICShareBasedPaymentArrangementDecreaseForCappedCalls_f4d2ea90-2057-41e0-ac98-32771fab04dc" xlink:href="mgni-20220630.xsd#mgni_APICShareBasedPaymentArrangementDecreaseForCappedCalls"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_mgni_APICShareBasedPaymentArrangementDecreaseForCappedCalls_f4d2ea90-2057-41e0-ac98-32771fab04dc" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_c6d009a9-903c-4ff0-bfa7-73acfb75498e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_c6d009a9-903c-4ff0-bfa7-73acfb75498e" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c4566202-d741-4cc2-9dcf-78945d81702a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_eb1dda62-0419-4d6a-9429-f889eca25f09" xlink:to="loc_us-gaap_NetIncomeLoss_c4566202-d741-4cc2-9dcf-78945d81702a" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_9fecf28b-4159-4693-b51d-c0262cfe6b4e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_639f9fc5-f467-4236-9e1f-20f26f6aa5b2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_f69e291b-35cb-4616-9a7e-5ef9bd0b5a0e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_10895eb2-9ea9-42d1-b726-eeedebf1c74b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_3f4cccbc-2de7-403e-80b1-a1a5af086aad" xlink:to="loc_us-gaap_StatementTable_10895eb2-9ea9-42d1-b726-eeedebf1c74b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_7a81278a-bd8e-492d-ac5c-fc99f8929119" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_10895eb2-9ea9-42d1-b726-eeedebf1c74b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_7a81278a-bd8e-492d-ac5c-fc99f8929119" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_7a81278a-bd8e-492d-ac5c-fc99f8929119_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7a81278a-bd8e-492d-ac5c-fc99f8929119" xlink:to="loc_us-gaap_EquityComponentDomain_7a81278a-bd8e-492d-ac5c-fc99f8929119_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7a81278a-bd8e-492d-ac5c-fc99f8929119" xlink:to="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_680b8058-b65b-4b0b-9548-f64efb996cff" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:to="loc_us-gaap_CommonStockMember_680b8058-b65b-4b0b-9548-f64efb996cff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_576afa8a-9222-434a-9d28-c07dcc13bda2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_576afa8a-9222-434a-9d28-c07dcc13bda2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_412b1e96-902f-428e-a02f-2332330fed4b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_412b1e96-902f-428e-a02f-2332330fed4b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_82b096fd-f9be-4e93-a2f6-58b985fa615e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:to="loc_us-gaap_RetainedEarningsMember_82b096fd-f9be-4e93-a2f6-58b985fa615e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_dc992aae-0f32-44cb-a8fa-aee590943e35" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1f32d8bb-6e65-4e77-bb48-ea916c7a3c9d" xlink:to="loc_us-gaap_TreasuryStockCommonMember_dc992aae-0f32-44cb-a8fa-aee590943e35" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetEarningsLossPerShareBasicandDilutedIncomeLossPerShareDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#NetEarningsLossPerShareBasicandDilutedIncomeLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetEarningsLossPerShareBasicandDilutedIncomeLossPerShareDetails" xlink:type="extended" id="ib160367c20bc46c6825e82da403db5c6_NetEarningsLossPerShareBasicandDilutedIncomeLossPerShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1f85166b-0a0a-49aa-bf39-ee75540924e1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1f85166b-0a0a-49aa-bf39-ee75540924e1" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_db1a4fa2-c907-44e7-9884-5c20c5019687" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:to="loc_us-gaap_NetIncomeLoss_db1a4fa2-c907-44e7-9884-5c20c5019687" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_7908bceb-1d99-4164-ac7a-d268e9067fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_7908bceb-1d99-4164-ac7a-d268e9067fdb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_0e3f7923-a8a0-429b-889f-13d4f2f3df0d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:to="loc_us-gaap_EarningsPerShareBasic_0e3f7923-a8a0-429b-889f-13d4f2f3df0d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_23f248f6-9a2a-4ce6-8247-48030c437d7b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_32c565ee-4994-4f9a-8819-ed22a36e4fca" xlink:to="loc_us-gaap_EarningsPerShareDiluted_23f248f6-9a2a-4ce6-8247-48030c437d7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_0fdc5df2-5dc1-4a88-9a2f-9d9fcca45899" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1f85166b-0a0a-49aa-bf39-ee75540924e1" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_0fdc5df2-5dc1-4a88-9a2f-9d9fcca45899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_30b52d4b-9b9a-4a45-9e6d-c56fd901f0a5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestOnConvertibleDebtNetOfTax_a7124758-509b-40f5-b24f-322e2fcf0fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestOnConvertibleDebtNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_0fdc5df2-5dc1-4a88-9a2f-9d9fcca45899" xlink:to="loc_us-gaap_InterestOnConvertibleDebtNetOfTax_a7124758-509b-40f5-b24f-322e2fcf0fbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_e2ac5f18-0628-4104-b081-9f233151ca45" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_0fdc5df2-5dc1-4a88-9a2f-9d9fcca45899" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_e2ac5f18-0628-4104-b081-9f233151ca45" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_0615a1b9-039e-455b-9015-270fa627870b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_0fdc5df2-5dc1-4a88-9a2f-9d9fcca45899" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_0615a1b9-039e-455b-9015-270fa627870b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_3d312a4c-f297-493b-a92f-0db1645f1f44" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_f2ada68c-750a-4b95-a426-3a80b2143b76" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_0615a1b9-039e-455b-9015-270fa627870b" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_f2ada68c-750a-4b95-a426-3a80b2143b76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_e89d7d00-86ea-4859-bf6c-3c66f4fd8736" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_0615a1b9-039e-455b-9015-270fa627870b" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_e89d7d00-86ea-4859-bf6c-3c66f4fd8736" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_23ad0571-088b-455c-a170-47501492d813" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_0615a1b9-039e-455b-9015-270fa627870b" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_23ad0571-088b-455c-a170-47501492d813" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_b26de8da-4d7a-4868-b779-69c8b3200f81" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_a7677589-0f20-43c3-9838-4204816cb74b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_1f85166b-0a0a-49aa-bf39-ee75540924e1" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_a7677589-0f20-43c3-9838-4204816cb74b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_a7677589-0f20-43c3-9838-4204816cb74b" xlink:to="loc_us-gaap_AwardTypeAxis_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5e7dee4a-f54e-44cd-b2b2-d33983e2e76f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_0b30bb7e-aac5-444b-aa73-71735760c79d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:to="loc_us-gaap_EmployeeStockOptionMember_0b30bb7e-aac5-444b-aa73-71735760c79d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_2e415074-0187-4c39-8361-31b505e8d468" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:to="loc_us-gaap_PerformanceSharesMember_2e415074-0187-4c39-8361-31b505e8d468" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_380fdc42-d855-470a-8439-fb6410ab6be1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_380fdc42-d855-470a-8439-fb6410ab6be1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_6d367603-0616-47ca-9ce3-76d4b7ca7776" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6d725b88-536e-4087-9787-1435e32a825a" xlink:to="loc_us-gaap_EmployeeStockMember_6d367603-0616-47ca-9ce3-76d4b7ca7776" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetEarningsLossPerShareSharesExcludedFromCalculationofDilutedNetLossPerShareDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#NetEarningsLossPerShareSharesExcludedFromCalculationofDilutedNetLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetEarningsLossPerShareSharesExcludedFromCalculationofDilutedNetLossPerShareDetails" xlink:type="extended" id="i9cd51178b20046d3a4169da5b9e6dfa6_NetEarningsLossPerShareSharesExcludedFromCalculationofDilutedNetLossPerShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ef86bbcd-5694-4ce2-9a05-85d1bc017820" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_7c9d89fe-ecba-4d40-ac10-e5e85597492f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ef86bbcd-5694-4ce2-9a05-85d1bc017820" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_7c9d89fe-ecba-4d40-ac10-e5e85597492f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_794b5f68-67c2-4c14-8d35-789263f3d282" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ef86bbcd-5694-4ce2-9a05-85d1bc017820" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_794b5f68-67c2-4c14-8d35-789263f3d282" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_f59556f2-2be9-46b3-9917-3c90a89aee72" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_794b5f68-67c2-4c14-8d35-789263f3d282" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_f59556f2-2be9-46b3-9917-3c90a89aee72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_f59556f2-2be9-46b3-9917-3c90a89aee72_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_f59556f2-2be9-46b3-9917-3c90a89aee72" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_f59556f2-2be9-46b3-9917-3c90a89aee72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_f59556f2-2be9-46b3-9917-3c90a89aee72" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockOptionMember_7727de9c-cc98-466f-ae56-55eafe02c831" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:to="loc_us-gaap_StockOptionMember_7727de9c-cc98-466f-ae56-55eafe02c831" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_61e0ac0b-d1d8-432a-8710-9b081777a2b5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_61e0ac0b-d1d8-432a-8710-9b081777a2b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_5bdb828a-0691-4250-af0a-25963c549b95" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:to="loc_us-gaap_PerformanceSharesMember_5bdb828a-0691-4250-af0a-25963c549b95" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_6e05a2ce-ed66-4eb2-b034-ff48d248f11a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:to="loc_us-gaap_EmployeeStockMember_6e05a2ce-ed66-4eb2-b034-ff48d248f11a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_93666c54-b232-4d13-9e0d-b615741eb49d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_abc8ca5c-7eb1-438f-bdca-8ce38c6dd1df" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_93666c54-b232-4d13-9e0d-b615741eb49d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetEarningsLossPerShareNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#NetEarningsLossPerShareNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetEarningsLossPerShareNarrativeDetails" xlink:type="extended" id="i0da9088df11947a390a6abf21bbcc0c1_NetEarningsLossPerShareNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae43c774-8bf7-4809-9cdc-bc62e80ce990" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_e2309758-52e1-4514-a074-5c53dac0c2a1" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae43c774-8bf7-4809-9cdc-bc62e80ce990" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_e2309758-52e1-4514-a074-5c53dac0c2a1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments_58be1587-e399-4ccd-9955-be1d9523cad8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae43c774-8bf7-4809-9cdc-bc62e80ce990" xlink:to="loc_us-gaap_DebtInstrumentConvertibleNumberOfEquityInstruments_58be1587-e399-4ccd-9955-be1d9523cad8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_25fd6cbe-9b2e-42af-813c-1d427896098c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae43c774-8bf7-4809-9cdc-bc62e80ce990" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_25fd6cbe-9b2e-42af-813c-1d427896098c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_71c80a00-4c71-4f30-bf51-8ad9a77b775d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_25fd6cbe-9b2e-42af-813c-1d427896098c" xlink:to="loc_us-gaap_AwardTypeAxis_71c80a00-4c71-4f30-bf51-8ad9a77b775d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71c80a00-4c71-4f30-bf51-8ad9a77b775d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_71c80a00-4c71-4f30-bf51-8ad9a77b775d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71c80a00-4c71-4f30-bf51-8ad9a77b775d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db74dc95-d5b9-4ce8-9162-6f39ac3e105d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_71c80a00-4c71-4f30-bf51-8ad9a77b775d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db74dc95-d5b9-4ce8-9162-6f39ac3e105d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db74dc95-d5b9-4ce8-9162-6f39ac3e105d" xlink:to="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedApril2020Member_856c14d8-51db-4cf8-ac3b-e344086c3230" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedApril2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:to="loc_mgni_PerformanceSharesGrantedApril2020Member_856c14d8-51db-4cf8-ac3b-e344086c3230" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedApril2021Member_e1cb6601-5f46-4ff9-bf1a-660ba6cef5c7" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedApril2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:to="loc_mgni_PerformanceSharesGrantedApril2021Member_e1cb6601-5f46-4ff9-bf1a-660ba6cef5c7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedAugust2021Member_825652bb-b1ec-4703-a764-5162f147063c" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedAugust2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:to="loc_mgni_PerformanceSharesGrantedAugust2021Member_825652bb-b1ec-4703-a764-5162f147063c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedFebruary2022Member_badedcc3-fdd6-4c5a-b9eb-b87685c275a4" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedFebruary2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_ec69e2bf-42cf-4e4c-bd6f-69dc90c370c7" xlink:to="loc_mgni_PerformanceSharesGrantedFebruary2022Member_badedcc3-fdd6-4c5a-b9eb-b87685c275a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_348f0dcc-8598-4d85-86f8-14a99b6beba6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_25fd6cbe-9b2e-42af-813c-1d427896098c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_348f0dcc-8598-4d85-86f8-14a99b6beba6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_348f0dcc-8598-4d85-86f8-14a99b6beba6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_348f0dcc-8598-4d85-86f8-14a99b6beba6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_348f0dcc-8598-4d85-86f8-14a99b6beba6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_41ddf1ae-e087-482d-9211-fbe35e793dcf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_348f0dcc-8598-4d85-86f8-14a99b6beba6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_41ddf1ae-e087-482d-9211-fbe35e793dcf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_fb4c003e-19b0-4a4d-88bc-3202a933e0f8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_41ddf1ae-e087-482d-9211-fbe35e793dcf" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_fb4c003e-19b0-4a4d-88bc-3202a933e0f8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueRevenueRecognizedonaGrossandNetBasisDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#RevenueRevenueRecognizedonaGrossandNetBasisDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueRevenueRecognizedonaGrossandNetBasisDetails" xlink:type="extended" id="i5dae92266610481ba61fd97c475df568_RevenueRevenueRecognizedonaGrossandNetBasisDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_30795985-f17b-4ce2-845e-bcb868656d52" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_15bb1bd4-07d3-4cbf-b06b-2e68b48e7d17" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_30795985-f17b-4ce2-845e-bcb868656d52" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_15bb1bd4-07d3-4cbf-b06b-2e68b48e7d17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_951618df-7dfc-46e9-8edf-1d5d00c5917e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_30795985-f17b-4ce2-845e-bcb868656d52" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_951618df-7dfc-46e9-8edf-1d5d00c5917e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_e1a58c0d-e4a4-4fb4-b297-7a429882685e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_30795985-f17b-4ce2-845e-bcb868656d52" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_e1a58c0d-e4a4-4fb4-b297-7a429882685e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_0049f309-f3a0-49d6-a0a7-852bf7de928a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_e1a58c0d-e4a4-4fb4-b297-7a429882685e" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_0049f309-f3a0-49d6-a0a7-852bf7de928a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_0049f309-f3a0-49d6-a0a7-852bf7de928a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_0049f309-f3a0-49d6-a0a7-852bf7de928a" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_0049f309-f3a0-49d6-a0a7-852bf7de928a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_152f57c1-5575-4680-8bf4-7c4c8a338ea0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_0049f309-f3a0-49d6-a0a7-852bf7de928a" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_152f57c1-5575-4680-8bf4-7c4c8a338ea0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_615ab851-fa47-40f7-9955-e35f3fdea455" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_152f57c1-5575-4680-8bf4-7c4c8a338ea0" xlink:to="loc_us-gaap_SalesRevenueNetMember_615ab851-fa47-40f7-9955-e35f3fdea455" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_c3bf08d7-11c8-4896-af42-131b219cf544" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_e1a58c0d-e4a4-4fb4-b297-7a429882685e" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_c3bf08d7-11c8-4896-af42-131b219cf544" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_c3bf08d7-11c8-4896-af42-131b219cf544_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_c3bf08d7-11c8-4896-af42-131b219cf544" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_c3bf08d7-11c8-4896-af42-131b219cf544_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_dde4cf58-b0be-4d44-8394-77c78ae65128" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_c3bf08d7-11c8-4896-af42-131b219cf544" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_dde4cf58-b0be-4d44-8394-77c78ae65128" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConcentrationOfBasisOfRevenueRecognitionMember_2ae52038-1f08-4491-a0b0-01a5fbdb8894" xlink:href="mgni-20220630.xsd#mgni_ConcentrationOfBasisOfRevenueRecognitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_dde4cf58-b0be-4d44-8394-77c78ae65128" xlink:to="loc_mgni_ConcentrationOfBasisOfRevenueRecognitionMember_2ae52038-1f08-4491-a0b0-01a5fbdb8894" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionAxis_4e6bba74-1b12-4215-9e7c-53e7c1735b29" xlink:href="mgni-20220630.xsd#mgni_BasisOfRevenueRecognitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_e1a58c0d-e4a4-4fb4-b297-7a429882685e" xlink:to="loc_mgni_BasisOfRevenueRecognitionAxis_4e6bba74-1b12-4215-9e7c-53e7c1735b29" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionDomain_4e6bba74-1b12-4215-9e7c-53e7c1735b29_default" xlink:href="mgni-20220630.xsd#mgni_BasisOfRevenueRecognitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_BasisOfRevenueRecognitionAxis_4e6bba74-1b12-4215-9e7c-53e7c1735b29" xlink:to="loc_mgni_BasisOfRevenueRecognitionDomain_4e6bba74-1b12-4215-9e7c-53e7c1735b29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionDomain_27d08fcc-3161-4b8a-9ce4-472f5dc6c957" xlink:href="mgni-20220630.xsd#mgni_BasisOfRevenueRecognitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_BasisOfRevenueRecognitionAxis_4e6bba74-1b12-4215-9e7c-53e7c1735b29" xlink:to="loc_mgni_BasisOfRevenueRecognitionDomain_27d08fcc-3161-4b8a-9ce4-472f5dc6c957" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NetRevenueMember_d6fe9602-8a33-467c-bc51-20c212df366a" xlink:href="mgni-20220630.xsd#mgni_NetRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_BasisOfRevenueRecognitionDomain_27d08fcc-3161-4b8a-9ce4-472f5dc6c957" xlink:to="loc_mgni_NetRevenueMember_d6fe9602-8a33-467c-bc51-20c212df366a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_GrossRevenueMember_f7cb38cf-b012-4bb8-adaa-470bda865482" xlink:href="mgni-20220630.xsd#mgni_GrossRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_BasisOfRevenueRecognitionDomain_27d08fcc-3161-4b8a-9ce4-472f5dc6c957" xlink:to="loc_mgni_GrossRevenueMember_f7cb38cf-b012-4bb8-adaa-470bda865482" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueRevenueDisaggregatedbySalesDistributionChannelDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#RevenueRevenueDisaggregatedbySalesDistributionChannelDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueRevenueDisaggregatedbySalesDistributionChannelDetails" xlink:type="extended" id="ife66405b5ec9423289eb79cbee21e185_RevenueRevenueDisaggregatedbySalesDistributionChannelDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_f5c3ffcd-0f39-4600-8998-6a89d11c9a99" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_4b7b0c77-9c72-49ec-b406-0ad761e4f634" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_f5c3ffcd-0f39-4600-8998-6a89d11c9a99" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_4b7b0c77-9c72-49ec-b406-0ad761e4f634" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_63645223-b30b-4bbf-a51a-13f42002667e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_f5c3ffcd-0f39-4600-8998-6a89d11c9a99" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_63645223-b30b-4bbf-a51a-13f42002667e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_b857b806-efa3-4896-aa38-81af32674057" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_f5c3ffcd-0f39-4600-8998-6a89d11c9a99" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_b857b806-efa3-4896-aa38-81af32674057" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_068f5c40-3eec-45e5-b82a-fb992adfabf4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b857b806-efa3-4896-aa38-81af32674057" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_068f5c40-3eec-45e5-b82a-fb992adfabf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_068f5c40-3eec-45e5-b82a-fb992adfabf4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_068f5c40-3eec-45e5-b82a-fb992adfabf4" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_068f5c40-3eec-45e5-b82a-fb992adfabf4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_d9ec0401-cd02-4fc0-ad6c-738c67a4796d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_068f5c40-3eec-45e5-b82a-fb992adfabf4" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_d9ec0401-cd02-4fc0-ad6c-738c67a4796d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductConcentrationRiskMember_04cd408d-7bc6-4278-b1a2-05ff35606b68" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProductConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_d9ec0401-cd02-4fc0-ad6c-738c67a4796d" xlink:to="loc_us-gaap_ProductConcentrationRiskMember_04cd408d-7bc6-4278-b1a2-05ff35606b68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6376f515-4979-40bb-870b-6e3929cf7106" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b857b806-efa3-4896-aa38-81af32674057" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6376f515-4979-40bb-870b-6e3929cf7106" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_6376f515-4979-40bb-870b-6e3929cf7106_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6376f515-4979-40bb-870b-6e3929cf7106" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_6376f515-4979-40bb-870b-6e3929cf7106_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_929a927b-f31e-4db6-8ca7-603dd7d7a70d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_6376f515-4979-40bb-870b-6e3929cf7106" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_929a927b-f31e-4db6-8ca7-603dd7d7a70d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_58532ef9-0952-4f9c-bde7-ff9c49390507" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_929a927b-f31e-4db6-8ca7-603dd7d7a70d" xlink:to="loc_us-gaap_SalesRevenueNetMember_58532ef9-0952-4f9c-bde7-ff9c49390507" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_b857b806-efa3-4896-aa38-81af32674057" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_31863279-f673-41c8-8557-4710694aafae" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_2d0746dd-1e14-4a48-b672-3b8ede8ddf3e" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_31863279-f673-41c8-8557-4710694aafae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConnectedTelevisionMember_728fc25a-af2c-4ce9-a2a8-8392c4e6fc73" xlink:href="mgni-20220630.xsd#mgni_ConnectedTelevisionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_31863279-f673-41c8-8557-4710694aafae" xlink:to="loc_mgni_ConnectedTelevisionMember_728fc25a-af2c-4ce9-a2a8-8392c4e6fc73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DesktopMember_6eee1847-eff7-41e8-8c59-e3ab4977263a" xlink:href="mgni-20220630.xsd#mgni_DesktopMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_31863279-f673-41c8-8557-4710694aafae" xlink:to="loc_mgni_DesktopMember_6eee1847-eff7-41e8-8c59-e3ab4977263a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_MobileMember_0bc943b7-a0e4-4233-a82c-58bf06cc4c92" xlink:href="mgni-20220630.xsd#mgni_MobileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_31863279-f673-41c8-8557-4710694aafae" xlink:to="loc_mgni_MobileMember_0bc943b7-a0e4-4233-a82c-58bf06cc4c92" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueRevenueDisaggregatedbyGeographicLocationDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#RevenueRevenueDisaggregatedbyGeographicLocationDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueRevenueDisaggregatedbyGeographicLocationDetails" xlink:type="extended" id="i00b8bc80aa0b4b7ead2f7e2e8cad61c9_RevenueRevenueDisaggregatedbyGeographicLocationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_1baea471-5897-4688-89b2-a31261bd16c2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_819ffb15-b0f5-4d70-b531-70a0fbe4dd95" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1baea471-5897-4688-89b2-a31261bd16c2" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_819ffb15-b0f5-4d70-b531-70a0fbe4dd95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_1aa0704d-0509-4c8d-991d-909855682802" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1baea471-5897-4688-89b2-a31261bd16c2" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_1aa0704d-0509-4c8d-991d-909855682802" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_693fef83-c75a-418b-8dd9-73b1bbf1ec22" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_1aa0704d-0509-4c8d-991d-909855682802" xlink:to="loc_srt_StatementGeographicalAxis_693fef83-c75a-418b-8dd9-73b1bbf1ec22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_693fef83-c75a-418b-8dd9-73b1bbf1ec22_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_693fef83-c75a-418b-8dd9-73b1bbf1ec22" xlink:to="loc_srt_SegmentGeographicalDomain_693fef83-c75a-418b-8dd9-73b1bbf1ec22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_ea1fd622-2b9c-4bed-9891-8129afc879b3" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_693fef83-c75a-418b-8dd9-73b1bbf1ec22" xlink:to="loc_srt_SegmentGeographicalDomain_ea1fd622-2b9c-4bed-9891-8129afc879b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_93d7d6c7-e7b7-4fff-b1fe-2fe27e388f26" xlink:href="https://xbrl.sec.gov/country/2022/country-2022.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ea1fd622-2b9c-4bed-9891-8129afc879b3" xlink:to="loc_country_US_93d7d6c7-e7b7-4fff-b1fe-2fe27e388f26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_05a83f59-19c7-4ea6-87cf-329efd409b42" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ea1fd622-2b9c-4bed-9891-8129afc879b3" xlink:to="loc_us-gaap_NonUsMember_05a83f59-19c7-4ea6-87cf-329efd409b42" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/FairValueMeasurementsFinancialInstrumentsDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#FairValueMeasurementsFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/FairValueMeasurementsFinancialInstrumentsDetails" xlink:type="extended" id="i3cd0f92b075f46d4970406d8659866b2_FairValueMeasurementsFinancialInstrumentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_68c56768-4f8d-473d-a9dd-3a40aee0dc67" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_0debb1d7-4dda-4bd8-9fea-0760481cd2fa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_68c56768-4f8d-473d-a9dd-3a40aee0dc67" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_0debb1d7-4dda-4bd8-9fea-0760481cd2fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_010642b3-3bcf-4289-8dcf-48ef5db38476" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_68c56768-4f8d-473d-a9dd-3a40aee0dc67" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_010642b3-3bcf-4289-8dcf-48ef5db38476" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_49af764f-de58-4ed6-ae6e-4ed530c4d90d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_010642b3-3bcf-4289-8dcf-48ef5db38476" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_49af764f-de58-4ed6-ae6e-4ed530c4d90d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_49af764f-de58-4ed6-ae6e-4ed530c4d90d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_49af764f-de58-4ed6-ae6e-4ed530c4d90d" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_49af764f-de58-4ed6-ae6e-4ed530c4d90d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_c8bb6acc-ca8c-4f31-8bd5-cfb74c8a4da5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_49af764f-de58-4ed6-ae6e-4ed530c4d90d" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_c8bb6acc-ca8c-4f31-8bd5-cfb74c8a4da5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_4b513320-58ae-4c1b-af04-0c27207d439b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_c8bb6acc-ca8c-4f31-8bd5-cfb74c8a4da5" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_4b513320-58ae-4c1b-af04-0c27207d439b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_464410e4-6581-44e2-8167-b7f17871c030" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_010642b3-3bcf-4289-8dcf-48ef5db38476" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_464410e4-6581-44e2-8167-b7f17871c030" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_464410e4-6581-44e2-8167-b7f17871c030_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_464410e4-6581-44e2-8167-b7f17871c030" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_464410e4-6581-44e2-8167-b7f17871c030_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f052796a-cda8-4965-abbe-b6774ebd29cc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_464410e4-6581-44e2-8167-b7f17871c030" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f052796a-cda8-4965-abbe-b6774ebd29cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_616a56ae-3c3b-41bc-89a2-34a560fb1dbb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f052796a-cda8-4965-abbe-b6774ebd29cc" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_616a56ae-3c3b-41bc-89a2-34a560fb1dbb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_83a60f51-fe1a-4800-a717-a5b2b221e15d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f052796a-cda8-4965-abbe-b6774ebd29cc" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_83a60f51-fe1a-4800-a717-a5b2b221e15d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_7365e871-f6d4-45d6-8927-1c4095b3dbde" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f052796a-cda8-4965-abbe-b6774ebd29cc" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_7365e871-f6d4-45d6-8927-1c4095b3dbde" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended" id="icb47fb97226c4683804200723e9cd30d_FairValueMeasurementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bc5c93a8-08a0-41ee-abb8-7843ced3ff2d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_7e3aae2d-249b-4954-ac12-fc39c8400b9f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bc5c93a8-08a0-41ee-abb8-7843ced3ff2d" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_7e3aae2d-249b-4954-ac12-fc39c8400b9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtFairValueDisclosures_905a31ed-42c9-4c8c-a898-1eb80c64fa8d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtFairValueDisclosures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bc5c93a8-08a0-41ee-abb8-7843ced3ff2d" xlink:to="loc_us-gaap_ConvertibleDebtFairValueDisclosures_905a31ed-42c9-4c8c-a898-1eb80c64fa8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_a5ff858d-93e6-48ba-a736-5ebaf023f11a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bc5c93a8-08a0-41ee-abb8-7843ced3ff2d" xlink:to="loc_us-gaap_LongTermDebtFairValue_a5ff858d-93e6-48ba-a736-5ebaf023f11a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bc5c93a8-08a0-41ee-abb8-7843ced3ff2d" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_c3de9791-b73d-4fc5-9565-6fb89ca40fd1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_c3de9791-b73d-4fc5-9565-6fb89ca40fd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_c3de9791-b73d-4fc5-9565-6fb89ca40fd1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_c3de9791-b73d-4fc5-9565-6fb89ca40fd1" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_c3de9791-b73d-4fc5-9565-6fb89ca40fd1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_c5b49949-ca09-47ef-aea1-691fb3891ea2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_c3de9791-b73d-4fc5-9565-6fb89ca40fd1" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_c5b49949-ca09-47ef-aea1-691fb3891ea2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_b3e8d45e-41db-47bd-932c-66a5e1d06054" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_c5b49949-ca09-47ef-aea1-691fb3891ea2" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_b3e8d45e-41db-47bd-932c-66a5e1d06054" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3cf5332-9eb3-427f-89c6-425557dda580" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3cf5332-9eb3-427f-89c6-425557dda580" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f3cf5332-9eb3-427f-89c6-425557dda580_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3cf5332-9eb3-427f-89c6-425557dda580" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f3cf5332-9eb3-427f-89c6-425557dda580_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_dd10f441-c85d-408e-a553-511f3bce3d9f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_f3cf5332-9eb3-427f-89c6-425557dda580" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_dd10f441-c85d-408e-a553-511f3bce3d9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_760876f1-2327-4e15-baa6-7fb98b0b7fbb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_dd10f441-c85d-408e-a553-511f3bce3d9f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_760876f1-2327-4e15-baa6-7fb98b0b7fbb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_4d19be9a-e85d-4445-9caf-e38f404bda58" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:to="loc_us-gaap_DebtInstrumentAxis_4d19be9a-e85d-4445-9caf-e38f404bda58" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4d19be9a-e85d-4445-9caf-e38f404bda58_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_4d19be9a-e85d-4445-9caf-e38f404bda58" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4d19be9a-e85d-4445-9caf-e38f404bda58_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c1d9ed27-ebe2-4ce3-8274-551ebb90b455" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_4d19be9a-e85d-4445-9caf-e38f404bda58" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c1d9ed27-ebe2-4ce3-8274-551ebb90b455" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_3f7813de-862c-4481-bf33-26b5b102b238" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c1d9ed27-ebe2-4ce3-8274-551ebb90b455" xlink:to="loc_us-gaap_SecuredDebtMember_3f7813de-862c-4481-bf33-26b5b102b238" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_ae7fe8fa-0651-49b6-b87e-82faa0cb4153" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_1dde9c01-b004-429f-a66d-d7c095c11b45" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_ae7fe8fa-0651-49b6-b87e-82faa0cb4153" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ae7fe8fa-0651-49b6-b87e-82faa0cb4153_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_ae7fe8fa-0651-49b6-b87e-82faa0cb4153" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ae7fe8fa-0651-49b6-b87e-82faa0cb4153_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_0657668c-7b53-4c81-8ac3-930e233a075c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_ae7fe8fa-0651-49b6-b87e-82faa0cb4153" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_0657668c-7b53-4c81-8ac3-930e233a075c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_f2438941-fd6e-483c-8b70-650ac6f9b259" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0657668c-7b53-4c81-8ac3-930e233a075c" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_f2438941-fd6e-483c-8b70-650ac6f9b259" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsNarrativeDetails" xlink:type="extended" id="i8d6b511ffe8a45e596fc274afb3cfa46_GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_921228d2-15b9-494a-9fc7-3a3f8b9def15" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c93323ec-7189-4900-811f-6ce8822fec44" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_921228d2-15b9-494a-9fc7-3a3f8b9def15" xlink:to="loc_us-gaap_Goodwill_c93323ec-7189-4900-811f-6ce8822fec44" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_e8f9ea19-da29-4ff4-a907-6d73f835722b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_921228d2-15b9-494a-9fc7-3a3f8b9def15" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_e8f9ea19-da29-4ff4-a907-6d73f835722b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwareNet_890c3fdc-d9b4-4919-bb6f-30f2b9bc82dd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalizedComputerSoftwareNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_921228d2-15b9-494a-9fc7-3a3f8b9def15" xlink:to="loc_us-gaap_CapitalizedComputerSoftwareNet_890c3fdc-d9b4-4919-bb6f-30f2b9bc82dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_d50dd0ab-1b77-4519-a29d-a9f3de66c450" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_921228d2-15b9-494a-9fc7-3a3f8b9def15" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_d50dd0ab-1b77-4519-a29d-a9f3de66c450" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_b32291d9-faf8-477b-b838-90817d76a4ec" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_d50dd0ab-1b77-4519-a29d-a9f3de66c450" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_b32291d9-faf8-477b-b838-90817d76a4ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_b32291d9-faf8-477b-b838-90817d76a4ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_b32291d9-faf8-477b-b838-90817d76a4ec" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_b32291d9-faf8-477b-b838-90817d76a4ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_78f7a66c-e1a6-4483-b871-7c55059160d6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_b32291d9-faf8-477b-b838-90817d76a4ec" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_78f7a66c-e1a6-4483-b871-7c55059160d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_84e32e95-a57d-4a73-a162-2f88b1727233" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_78f7a66c-e1a6-4483-b871-7c55059160d6" xlink:to="loc_us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember_84e32e95-a57d-4a73-a162-2f88b1727233" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_906860a6-7aa0-4810-b653-b7d5c31be70d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_78f7a66c-e1a6-4483-b871-7c55059160d6" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_906860a6-7aa0-4810-b653-b7d5c31be70d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsFiniteLivedIntangibleAssetsDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsFiniteLivedIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsFiniteLivedIntangibleAssetsDetails" xlink:type="extended" id="i222b6ca0fbcc4551933c377a27b2254b_GoodwillIntangibleAssetsandCapitalizedCostsIncurredinCloudComputingArrangementsFiniteLivedIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_02fee4ce-d487-47db-b649-5c54efbeeb29" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_3694e2ef-3ae1-45b2-9e00-a09045bca808" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_02fee4ce-d487-47db-b649-5c54efbeeb29" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_3694e2ef-3ae1-45b2-9e00-a09045bca808" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_c9cc649e-a251-4a74-89a7-1b9b4c3f7b2d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_02fee4ce-d487-47db-b649-5c54efbeeb29" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_c9cc649e-a251-4a74-89a7-1b9b4c3f7b2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_72f59888-5541-4c65-afcb-ac88bf23c1a4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_02fee4ce-d487-47db-b649-5c54efbeeb29" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_72f59888-5541-4c65-afcb-ac88bf23c1a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_693333ff-6101-4530-b53a-8aa81cf9a10f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_02fee4ce-d487-47db-b649-5c54efbeeb29" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_693333ff-6101-4530-b53a-8aa81cf9a10f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ed79ecd3-f28e-45f9-a25b-1656e355cb4f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_693333ff-6101-4530-b53a-8aa81cf9a10f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ed79ecd3-f28e-45f9-a25b-1656e355cb4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ed79ecd3-f28e-45f9-a25b-1656e355cb4f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ed79ecd3-f28e-45f9-a25b-1656e355cb4f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ed79ecd3-f28e-45f9-a25b-1656e355cb4f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ed79ecd3-f28e-45f9-a25b-1656e355cb4f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_f81161fa-39b4-4fa5-863a-5338c040d330" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_f81161fa-39b4-4fa5-863a-5338c040d330" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_c2107066-74d7-4b1e-b25b-429d36c3f677" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:to="loc_us-gaap_CustomerRelationshipsMember_c2107066-74d7-4b1e-b25b-429d36c3f677" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_7987b1f2-87c0-45f2-9a62-e193e26196e5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_7987b1f2-87c0-45f2-9a62-e193e26196e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_b55c04f6-af23-4e61-8358-ea155359e657" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_b55c04f6-af23-4e61-8358-ea155359e657" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_9e0ed79f-24f4-4cd2-851c-7bda4376f29b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d070d68b-16d2-4071-af65-3d93d4a91430" xlink:to="loc_us-gaap_TrademarksMember_9e0ed79f-24f4-4cd2-851c-7bda4376f29b" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsNarrativeDetails" xlink:type="extended" id="icde4e5ab2c16484ea12ecbf4ac5d12f0_BusinessCombinationsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_1c11f399-b911-4aa1-99a1-9f91927f7905" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_1c11f399-b911-4aa1-99a1-9f91927f7905" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_5ecb3f03-9881-4b12-b8e4-da41d8ac22d5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_5ecb3f03-9881-4b12-b8e4-da41d8ac22d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PaymentsToAcquireBusinessesGrossElectedIncreasePercentage_49f41a38-52bf-4d06-9830-dd5f1097efa2" xlink:href="mgni-20220630.xsd#mgni_PaymentsToAcquireBusinessesGrossElectedIncreasePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_PaymentsToAcquireBusinessesGrossElectedIncreasePercentage_49f41a38-52bf-4d06-9830-dd5f1097efa2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PaymentsToAcquireBusinessesGrossElectedIncreaseAmount_ac9fa7aa-6dd8-4ee1-bcd8-1e6bd36ebecb" xlink:href="mgni-20220630.xsd#mgni_PaymentsToAcquireBusinessesGrossElectedIncreaseAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_PaymentsToAcquireBusinessesGrossElectedIncreaseAmount_ac9fa7aa-6dd8-4ee1-bcd8-1e6bd36ebecb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreasePercentage_a7c41ca8-2387-4694-8960-eda9274955b1" xlink:href="mgni-20220630.xsd#mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreasePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreasePercentage_a7c41ca8-2387-4694-8960-eda9274955b1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreaseValue_00d77d1e-a742-4069-b429-5e546f6ebe19" xlink:href="mgni-20220630.xsd#mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreaseValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_BusinessAcquisitionEquityInterestIssuedOrIssuableElectedDecreaseValue_00d77d1e-a742-4069-b429-5e546f6ebe19" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_85e09818-1abf-4fff-ae06-342b61819b3e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_SharePrice_85e09818-1abf-4fff-ae06-342b61819b3e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationConsiderationTransferredBeforeWorkingCapitalAdjustments_c7586061-d383-411a-8338-20a646b91fd7" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationConsiderationTransferredBeforeWorkingCapitalAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_BusinessCombinationConsiderationTransferredBeforeWorkingCapitalAdjustments_c7586061-d383-411a-8338-20a646b91fd7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_ae868391-9075-47bd-a435-3051e5ab7346" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_ae868391-9075-47bd-a435-3051e5ab7346" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_4101f9b0-1e9f-4839-b16e-d81638cc9acd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_4101f9b0-1e9f-4839-b16e-d81638cc9acd" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_f5e7ec35-81b5-4f27-8c2a-4739c30b0cea" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_f5e7ec35-81b5-4f27-8c2a-4739c30b0cea" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_ac5839ee-a48a-414e-b651-f077f81b9b6f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_ac5839ee-a48a-414e-b651-f077f81b9b6f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_14ab7cf4-eb19-410a-b366-d2dd939da49a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_14ab7cf4-eb19-410a-b366-d2dd939da49a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVenturesFairValueDisclosure_b8de1f88-8d3b-4709-b27d-d8ffedffa33f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVenturesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVenturesFairValueDisclosure_b8de1f88-8d3b-4709-b27d-d8ffedffa33f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationIndemnificationAssetsAmountAsOfAcquisitionDate_878788ae-bb79-4212-bac3-28f5cd5680df" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationIndemnificationAssetsAmountAsOfAcquisitionDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessCombinationIndemnificationAssetsAmountAsOfAcquisitionDate_878788ae-bb79-4212-bac3-28f5cd5680df" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_cbd421c4-38a4-42e5-ac92-89f9d4a4137e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_cbd421c4-38a4-42e5-ac92-89f9d4a4137e" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_523cfecb-3e87-4a56-8d08-9765cf589ea3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_523cfecb-3e87-4a56-8d08-9765cf589ea3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_faecd1f1-eeb7-4f05-a846-de84eddacedb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_faecd1f1-eeb7-4f05-a846-de84eddacedb" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_ead40d8f-9173-4e26-b927-ad3380c7773d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_ead40d8f-9173-4e26-b927-ad3380c7773d" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c4ba853b-bbd6-4046-90a7-3ed3fcf262cd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_Goodwill_c4ba853b-bbd6-4046-90a7-3ed3fcf262cd" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_3f6eefc4-76fb-4872-b4ba-68a552d22d44" xlink:href="mgni-20220630.xsd#mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_3f6eefc4-76fb-4872-b4ba-68a552d22d44" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_defe90b5-0d6b-4155-804b-eaeabd1fc66a" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_31eb986e-0617-473e-9749-1390993dc6b6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_31eb986e-0617-473e-9749-1390993dc6b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_31eb986e-0617-473e-9749-1390993dc6b6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_31eb986e-0617-473e-9749-1390993dc6b6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_31eb986e-0617-473e-9749-1390993dc6b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f993e495-f4ef-408b-bccb-538f690a4750" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_31eb986e-0617-473e-9749-1390993dc6b6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f993e495-f4ef-408b-bccb-538f690a4750" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncAndServeMotionIncMember_747e6f25-725c-4c4e-8b56-5173e835d97b" xlink:href="mgni-20220630.xsd#mgni_SpotXIncAndServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f993e495-f4ef-408b-bccb-538f690a4750" xlink:to="loc_mgni_SpotXIncAndServeMotionIncMember_747e6f25-725c-4c4e-8b56-5173e835d97b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncMember_e14c9133-b329-48c0-8c61-4159192ddfa5" xlink:href="mgni-20220630.xsd#mgni_SpotXIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_SpotXIncAndServeMotionIncMember_747e6f25-725c-4c4e-8b56-5173e835d97b" xlink:to="loc_mgni_SpotXIncMember_e14c9133-b329-48c0-8c61-4159192ddfa5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ServeMotionIncMember_7cbd7d48-af4b-4f6e-82fd-6ae37615c8a6" xlink:href="mgni-20220630.xsd#mgni_ServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_SpotXIncAndServeMotionIncMember_747e6f25-725c-4c4e-8b56-5173e835d97b" xlink:to="loc_mgni_ServeMotionIncMember_7cbd7d48-af4b-4f6e-82fd-6ae37615c8a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NthPartyLtdMember_b97e5f78-f056-4562-bfab-db2c3f3a4f14" xlink:href="mgni-20220630.xsd#mgni_NthPartyLtdMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f993e495-f4ef-408b-bccb-538f690a4750" xlink:to="loc_mgni_NthPartyLtdMember_b97e5f78-f056-4562-bfab-db2c3f3a4f14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CarbonAILimitedAndModeTechnologiesIncMember_a10e3c22-9258-44dd-b98a-d5b50422de14" xlink:href="mgni-20220630.xsd#mgni_CarbonAILimitedAndModeTechnologiesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f993e495-f4ef-408b-bccb-538f690a4750" xlink:to="loc_mgni_CarbonAILimitedAndModeTechnologiesIncMember_a10e3c22-9258-44dd-b98a-d5b50422de14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_18596e45-2df2-4c80-89f2-a9d6a6863dbd" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:to="loc_dei_LegalEntityAxis_18596e45-2df2-4c80-89f2-a9d6a6863dbd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_18596e45-2df2-4c80-89f2-a9d6a6863dbd_default" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_18596e45-2df2-4c80-89f2-a9d6a6863dbd" xlink:to="loc_dei_EntityDomain_18596e45-2df2-4c80-89f2-a9d6a6863dbd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_19802f5b-96cf-4112-968c-29ab1facb29d" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_18596e45-2df2-4c80-89f2-a9d6a6863dbd" xlink:to="loc_dei_EntityDomain_19802f5b-96cf-4112-968c-29ab1facb29d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncMember_5813408f-a86f-4cb6-b1f1-a2adbd0c638d" xlink:href="mgni-20220630.xsd#mgni_SpotXIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_19802f5b-96cf-4112-968c-29ab1facb29d" xlink:to="loc_mgni_SpotXIncMember_5813408f-a86f-4cb6-b1f1-a2adbd0c638d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9cb12fe5-59f1-41a0-bf7f-54011e914e25" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9cb12fe5-59f1-41a0-bf7f-54011e914e25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_9cb12fe5-59f1-41a0-bf7f-54011e914e25_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9cb12fe5-59f1-41a0-bf7f-54011e914e25" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_9cb12fe5-59f1-41a0-bf7f-54011e914e25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_49ace681-cef1-4543-a1b9-500c282157f6" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9cb12fe5-59f1-41a0-bf7f-54011e914e25" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_49ace681-cef1-4543-a1b9-500c282157f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ServeMotionIncMember_516b975d-a811-401a-afe7-c463ab86ca9a" xlink:href="mgni-20220630.xsd#mgni_ServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_49ace681-cef1-4543-a1b9-500c282157f6" xlink:to="loc_mgni_ServeMotionIncMember_516b975d-a811-401a-afe7-c463ab86ca9a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8a249af6-b19a-4b51-9a04-b127ebc4a908" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3bd96e59-277a-4cd2-a385-4c346af10366" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8a249af6-b19a-4b51-9a04-b127ebc4a908" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8a249af6-b19a-4b51-9a04-b127ebc4a908_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8a249af6-b19a-4b51-9a04-b127ebc4a908" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8a249af6-b19a-4b51-9a04-b127ebc4a908_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_028a49a3-6d9d-4abf-ad46-2976884d6a2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8a249af6-b19a-4b51-9a04-b127ebc4a908" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_028a49a3-6d9d-4abf-ad46-2976884d6a2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_6d727c4a-21a5-4ef2-b0f0-4c7eb3d6fb64" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_028a49a3-6d9d-4abf-ad46-2976884d6a2e" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_6d727c4a-21a5-4ef2-b0f0-4c7eb3d6fb64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_714d82df-9757-4d9c-a8c3-850c53be009a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_028a49a3-6d9d-4abf-ad46-2976884d6a2e" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_714d82df-9757-4d9c-a8c3-850c53be009a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_7d2f8cb8-5452-42b8-81f5-c1c3c099b905" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_028a49a3-6d9d-4abf-ad46-2976884d6a2e" xlink:to="loc_us-gaap_CustomerRelationshipsMember_7d2f8cb8-5452-42b8-81f5-c1c3c099b905" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpotXPurchaseConsiderationDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpotXPurchaseConsiderationDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpotXPurchaseConsiderationDetails" xlink:type="extended" id="i6ad22a411e2f4de781d632d70a31bb65_BusinessCombinationsSpotXPurchaseConsiderationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_3694acb5-28c5-4c42-ac04-bd0f349bb888" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_3694acb5-28c5-4c42-ac04-bd0f349bb888" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_e5bd1606-1fea-49df-873a-5fde06dc0144" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_e5bd1606-1fea-49df-873a-5fde06dc0144" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_c6f219bf-83f0-4a6e-8e11-f3661a294c42" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:to="loc_mgni_BusinessCombinationConsiderationTransferredWorkingCapitalAdjustments_c6f219bf-83f0-4a6e-8e11-f3661a294c42" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_b514ca26-03cf-4231-9d53-8dd86e5811a0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_b514ca26-03cf-4231-9d53-8dd86e5811a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dfe0380e-7f06-4bc6-9768-bea426f02d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_c641acc4-9f0a-4946-8ef9-b40f86d1978e" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dfe0380e-7f06-4bc6-9768-bea426f02d1e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_2279c0a7-e7bd-4318-ae92-b4e30f796738" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dfe0380e-7f06-4bc6-9768-bea426f02d1e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_2279c0a7-e7bd-4318-ae92-b4e30f796738" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2279c0a7-e7bd-4318-ae92-b4e30f796738_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2279c0a7-e7bd-4318-ae92-b4e30f796738" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2279c0a7-e7bd-4318-ae92-b4e30f796738_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3e066bb1-42ba-4162-bd8c-1a34b7975677" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2279c0a7-e7bd-4318-ae92-b4e30f796738" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3e066bb1-42ba-4162-bd8c-1a34b7975677" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncMember_b11cc5ae-275b-4217-adb1-994472d4f9b6" xlink:href="mgni-20220630.xsd#mgni_SpotXIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_3e066bb1-42ba-4162-bd8c-1a34b7975677" xlink:to="loc_mgni_SpotXIncMember_b11cc5ae-275b-4217-adb1-994472d4f9b6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpotXScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpotXScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpotXScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="extended" id="i77a20fcdeb924854809d5250c13911cb_BusinessCombinationsSpotXScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b355bcbf-87a8-4762-8f53-b652c1d4ad19" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_b355bcbf-87a8-4762-8f53-b652c1d4ad19" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash_ef012918-e315-486d-8c0f-bb91f1079a6c" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRestrictedCash_ef012918-e315-486d-8c0f-bb91f1079a6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsAccountsReceivables_9a91e4ec-c825-46a4-ac9c-9f17ed2979d2" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsAccountsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsAccountsReceivables_9a91e4ec-c825-46a4-ac9c-9f17ed2979d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_0674a7bb-3e22-48ff-b0cb-684236d3399c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_0674a7bb-3e22-48ff-b0cb-684236d3399c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_7f1ecd67-a8ba-42b1-96ee-522f78f553eb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_7f1ecd67-a8ba-42b1-96ee-522f78f553eb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_5deb6d80-44a8-4a4b-a8fd-42ac7a36cd42" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_5deb6d80-44a8-4a4b-a8fd-42ac7a36cd42" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset_df7900bf-5249-420e-a00f-23f239b56394" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset_df7900bf-5249-420e-a00f-23f239b56394" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_e8928afa-e7e9-473a-a511-96bb22edd7d0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_Goodwill_e8928afa-e7e9-473a-a511-96bb22edd7d0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_39314b83-0eb5-4a45-9d72-b45974aabe34" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_39314b83-0eb5-4a45-9d72-b45974aabe34" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities_b00bc82e-bdb4-47a3-a594-c60066523c02" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities_b00bc82e-bdb4-47a3-a594-c60066523c02" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_652b3a2d-7229-4731-8196-c1c0582744c6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_652b3a2d-7229-4731-8196-c1c0582744c6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_397c4663-a7ae-4803-a501-465489966da7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_397c4663-a7ae-4803-a501-465489966da7" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_4541baff-b744-4ca8-821e-2013ef0e6585" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_4541baff-b744-4ca8-821e-2013ef0e6585" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_f4844334-0c4b-49b1-b23a-0dba41465812" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_f4844334-0c4b-49b1-b23a-0dba41465812" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_bc4efa21-6be7-4ee7-bfd4-573d48b5ba2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_bc4efa21-6be7-4ee7-bfd4-573d48b5ba2e" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b51ce315-03a4-43df-a3f6-8451f9612fc5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_e20b43cc-db3e-480e-aebd-244f0c05a890" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b51ce315-03a4-43df-a3f6-8451f9612fc5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_69386edd-e805-41bd-9ab4-35f7487a12ef" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b51ce315-03a4-43df-a3f6-8451f9612fc5" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_69386edd-e805-41bd-9ab4-35f7487a12ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_69386edd-e805-41bd-9ab4-35f7487a12ef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_69386edd-e805-41bd-9ab4-35f7487a12ef" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_69386edd-e805-41bd-9ab4-35f7487a12ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c9bf76d9-bbd7-43c3-9771-30391372ce71" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_69386edd-e805-41bd-9ab4-35f7487a12ef" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c9bf76d9-bbd7-43c3-9771-30391372ce71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncMember_6c22ef09-8508-4548-9cf7-232c5fe82871" xlink:href="mgni-20220630.xsd#mgni_SpotXIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c9bf76d9-bbd7-43c3-9771-30391372ce71" xlink:to="loc_mgni_SpotXIncMember_6c22ef09-8508-4548-9cf7-232c5fe82871" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpotXComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpotXComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpotXComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails" xlink:type="extended" id="if86891aa47ae486c9d37d02f5b81fce6_BusinessCombinationsSpotXComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_db30a5c1-8184-4a4c-aceb-52812ecfc2c2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_c89adf77-4475-47b0-9b07-1110fba8c6b9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_db30a5c1-8184-4a4c-aceb-52812ecfc2c2" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_c89adf77-4475-47b0-9b07-1110fba8c6b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NumberOfProjects_b3aa12d1-a821-4b5c-8768-e67f0c3aabc1" xlink:href="mgni-20220630.xsd#mgni_NumberOfProjects"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_db30a5c1-8184-4a4c-aceb-52812ecfc2c2" xlink:to="loc_mgni_NumberOfProjects_b3aa12d1-a821-4b5c-8768-e67f0c3aabc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_52667e0b-f635-45cc-beeb-5ef8f515a269" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_db30a5c1-8184-4a4c-aceb-52812ecfc2c2" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_52667e0b-f635-45cc-beeb-5ef8f515a269" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f8675e9c-1450-4f75-85f2-c57143ee991f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_db30a5c1-8184-4a4c-aceb-52812ecfc2c2" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f8675e9c-1450-4f75-85f2-c57143ee991f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_203cbf1d-ab95-42cb-b4ec-ed467cde98b7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f8675e9c-1450-4f75-85f2-c57143ee991f" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_203cbf1d-ab95-42cb-b4ec-ed467cde98b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_203cbf1d-ab95-42cb-b4ec-ed467cde98b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_203cbf1d-ab95-42cb-b4ec-ed467cde98b7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_203cbf1d-ab95-42cb-b4ec-ed467cde98b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9ad829b8-9a39-4b73-9805-956f1389264d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_203cbf1d-ab95-42cb-b4ec-ed467cde98b7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9ad829b8-9a39-4b73-9805-956f1389264d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncMember_2e04629e-9e6c-4580-a0e4-0df8b908b541" xlink:href="mgni-20220630.xsd#mgni_SpotXIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9ad829b8-9a39-4b73-9805-956f1389264d" xlink:to="loc_mgni_SpotXIncMember_2e04629e-9e6c-4580-a0e4-0df8b908b541" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_67379745-dd59-4f1b-91d7-0f58796814f0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f8675e9c-1450-4f75-85f2-c57143ee991f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_67379745-dd59-4f1b-91d7-0f58796814f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_67379745-dd59-4f1b-91d7-0f58796814f0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_67379745-dd59-4f1b-91d7-0f58796814f0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_67379745-dd59-4f1b-91d7-0f58796814f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_67379745-dd59-4f1b-91d7-0f58796814f0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_c2240615-aa14-4af2-8046-7e00e77c278e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_c2240615-aa14-4af2-8046-7e00e77c278e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_47ea3bca-9e9c-4a62-80e7-0cd5d1b14ff0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_CustomerRelationshipsMember_47ea3bca-9e9c-4a62-80e7-0cd5d1b14ff0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_f3ea34fe-50f3-412e-9f7f-afc016a5e543" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_f3ea34fe-50f3-412e-9f7f-afc016a5e543" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_c03fef63-240f-4814-aa27-771f82b64035" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_c03fef63-240f-4814-aa27-771f82b64035" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_5629e2a2-6a59-4b37-8f23-82cd9a3ed536" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_5629e2a2-6a59-4b37-8f23-82cd9a3ed536" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_6055ba21-d4cc-439b-aa09-510fc049d8f9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_481cf79e-726b-4a23-9e1e-2f8e1838c767" xlink:to="loc_us-gaap_TrademarksMember_6055ba21-d4cc-439b-aa09-510fc049d8f9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_6884faec-0a3d-4d6e-bc2b-d324f8d015bb" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f8675e9c-1450-4f75-85f2-c57143ee991f" xlink:to="loc_srt_RangeAxis_6884faec-0a3d-4d6e-bc2b-d324f8d015bb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_6884faec-0a3d-4d6e-bc2b-d324f8d015bb_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_6884faec-0a3d-4d6e-bc2b-d324f8d015bb" xlink:to="loc_srt_RangeMember_6884faec-0a3d-4d6e-bc2b-d324f8d015bb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2b3b91a6-987c-4d20-87c7-f1ce74587064" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_6884faec-0a3d-4d6e-bc2b-d324f8d015bb" xlink:to="loc_srt_RangeMember_2b3b91a6-987c-4d20-87c7-f1ce74587064" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_802d40a8-d062-4145-800f-eaccb9ad4af9" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2b3b91a6-987c-4d20-87c7-f1ce74587064" xlink:to="loc_srt_MinimumMember_802d40a8-d062-4145-800f-eaccb9ad4af9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_fc224950-a160-4e8e-a830-01dd25f3cdc5" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2b3b91a6-987c-4d20-87c7-f1ce74587064" xlink:to="loc_srt_MaximumMember_fc224950-a160-4e8e-a830-01dd25f3cdc5" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpringServePurchaseConsiderationDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpringServePurchaseConsiderationDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpringServePurchaseConsiderationDetails" xlink:type="extended" id="i879aa1d57f0143d182b92858bed4b6bf_BusinessCombinationsSpringServePurchaseConsiderationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_d73e8eca-cc8e-49dc-ba64-1c71c71253df" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_d73e8eca-cc8e-49dc-ba64-1c71c71253df" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeFairValue1_a966c96a-c794-4707-af58-86f35123aa4d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeFairValue1_a966c96a-c794-4707-af58-86f35123aa4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain_795f8b68-0416-4da5-be0d-f72d8a5c3531" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain_795f8b68-0416-4da5-be0d-f72d8a5c3531" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback_fb0a06a2-63b0-4ce2-99f9-82fae3855125" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback_fb0a06a2-63b0-4ce2-99f9-82fae3855125" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredIncludingEquityInterestInAcquireeHeldPriorToCombination1_10a820cb-808a-46c6-97d0-1e280a86fe79" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationConsiderationTransferredIncludingEquityInterestInAcquireeHeldPriorToCombination1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredIncludingEquityInterestInAcquireeHeldPriorToCombination1_10a820cb-808a-46c6-97d0-1e280a86fe79" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85f75204-b913-4ce4-9078-a7e55ea0a5c7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2d076b0a-0c9d-43fb-bd55-ea3bf7c68190" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85f75204-b913-4ce4-9078-a7e55ea0a5c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_d34444e1-0885-48b9-971f-bca79f97338a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85f75204-b913-4ce4-9078-a7e55ea0a5c7" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_d34444e1-0885-48b9-971f-bca79f97338a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d34444e1-0885-48b9-971f-bca79f97338a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_d34444e1-0885-48b9-971f-bca79f97338a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d34444e1-0885-48b9-971f-bca79f97338a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8d385171-0103-4fb0-b6c5-e995db80bed4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_d34444e1-0885-48b9-971f-bca79f97338a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8d385171-0103-4fb0-b6c5-e995db80bed4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ServeMotionIncMember_ae2baeaa-1f80-49ae-bd4b-4a70e9f7bad9" xlink:href="mgni-20220630.xsd#mgni_ServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8d385171-0103-4fb0-b6c5-e995db80bed4" xlink:to="loc_mgni_ServeMotionIncMember_ae2baeaa-1f80-49ae-bd4b-4a70e9f7bad9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpringServeScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpringServeScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpringServeScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="extended" id="i58cb48f09ce7422384f63289a7f3f10a_BusinessCombinationsSpringServeScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_df6179a8-fe22-4245-9460-d93d24d734b4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_df6179a8-fe22-4245-9460-d93d24d734b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_c4da83ba-9e71-4fd4-b30d-ebdbd5afc4c5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_c4da83ba-9e71-4fd4-b30d-ebdbd5afc4c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_508ac26d-277b-4188-ab1f-56eac59a396d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_508ac26d-277b-4188-ab1f-56eac59a396d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_f3e9bea2-3fa7-4f02-a80a-8e960c2d6502" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_f3e9bea2-3fa7-4f02-a80a-8e960c2d6502" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_213a3268-2f22-4fb5-a8e5-e459dc6a8d94" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_213a3268-2f22-4fb5-a8e5-e459dc6a8d94" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset_fc1f661f-063d-440b-80f1-458da185466d" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRightOfUseLeaseAsset_fc1f661f-063d-440b-80f1-458da185466d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_581acd58-b5a1-4a1c-83d6-0f665490ead6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_Goodwill_581acd58-b5a1-4a1c-83d6-0f665490ead6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_4c6f2a36-5ff7-4f9f-a13d-cc7247b6aa9b" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAcquiredIncludingGoodwill_4c6f2a36-5ff7-4f9f-a13d-cc7247b6aa9b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities_452381b4-8528-47db-81e7-19eb695b9874" xlink:href="mgni-20220630.xsd#mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_mgni_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAccountsPayableAndAccruedLiabilities_452381b4-8528-47db-81e7-19eb695b9874" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_007fd7a9-6061-4651-b892-d584e59b8955" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_007fd7a9-6061-4651-b892-d584e59b8955" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_5c410420-6d45-4c11-a2b7-32aeb5fcbbef" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCapitalLeaseObligation_5c410420-6d45-4c11-a2b7-32aeb5fcbbef" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_7c9bbff7-9601-4989-b05b-cd28ac70f95f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_7c9bbff7-9601-4989-b05b-cd28ac70f95f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_e8d82282-8b36-4369-90aa-2ab229adbe72" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_e8d82282-8b36-4369-90aa-2ab229adbe72" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_967b5459-bde9-473e-8aa2-ca053a02bca6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_967b5459-bde9-473e-8aa2-ca053a02bca6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f2754a8e-996f-4a0e-96f8-58cb8045fec7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a82d78d5-70d7-4eb2-a2f9-78d0095d1d2b" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f2754a8e-996f-4a0e-96f8-58cb8045fec7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9f95cc42-4597-44e3-b368-b1fec8b8513b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_f2754a8e-996f-4a0e-96f8-58cb8045fec7" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9f95cc42-4597-44e3-b368-b1fec8b8513b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9f95cc42-4597-44e3-b368-b1fec8b8513b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9f95cc42-4597-44e3-b368-b1fec8b8513b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9f95cc42-4597-44e3-b368-b1fec8b8513b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ddea105b-db45-401b-86b8-501a30cd681b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9f95cc42-4597-44e3-b368-b1fec8b8513b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ddea105b-db45-401b-86b8-501a30cd681b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ServeMotionIncMember_8a6dd47f-b092-4fbc-bc44-efff6436e1af" xlink:href="mgni-20220630.xsd#mgni_ServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ddea105b-db45-401b-86b8-501a30cd681b" xlink:to="loc_mgni_ServeMotionIncMember_8a6dd47f-b092-4fbc-bc44-efff6436e1af" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpringServeComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpringServeComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpringServeComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails" xlink:type="extended" id="i729fe9268c784793aad87e9cc488d990_BusinessCombinationsSpringServeComponentsofIntangibleAssetsandEstimatedUsefulLivesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_43e6b93f-8e6e-4cd4-a3d3-68ad2114333d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_0c7b6b05-e1d0-491c-9643-149d90396e7d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_43e6b93f-8e6e-4cd4-a3d3-68ad2114333d" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_0c7b6b05-e1d0-491c-9643-149d90396e7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_26d74551-791e-45f9-a8af-d80e8cfc6c00" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_43e6b93f-8e6e-4cd4-a3d3-68ad2114333d" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_26d74551-791e-45f9-a8af-d80e8cfc6c00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NumberOfProjects_d8d17a48-8d9e-4ae8-956e-8536ad7b5503" xlink:href="mgni-20220630.xsd#mgni_NumberOfProjects"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_43e6b93f-8e6e-4cd4-a3d3-68ad2114333d" xlink:to="loc_mgni_NumberOfProjects_d8d17a48-8d9e-4ae8-956e-8536ad7b5503" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85d60814-21ec-4aac-a192-39bae9817f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_43e6b93f-8e6e-4cd4-a3d3-68ad2114333d" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85d60814-21ec-4aac-a192-39bae9817f2a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0c42c936-8ec6-4d7f-a172-fa2621ef3290" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85d60814-21ec-4aac-a192-39bae9817f2a" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0c42c936-8ec6-4d7f-a172-fa2621ef3290" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0c42c936-8ec6-4d7f-a172-fa2621ef3290_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0c42c936-8ec6-4d7f-a172-fa2621ef3290" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0c42c936-8ec6-4d7f-a172-fa2621ef3290_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f30595b1-d5ea-417e-84e9-78a602825e2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0c42c936-8ec6-4d7f-a172-fa2621ef3290" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f30595b1-d5ea-417e-84e9-78a602825e2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ServeMotionIncMember_0c54b465-ca1d-472b-818f-f4bd4636630d" xlink:href="mgni-20220630.xsd#mgni_ServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f30595b1-d5ea-417e-84e9-78a602825e2e" xlink:to="loc_mgni_ServeMotionIncMember_0c54b465-ca1d-472b-818f-f4bd4636630d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_02d12057-10c7-45f3-bb34-427c0f3b6ad5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_85d60814-21ec-4aac-a192-39bae9817f2a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_02d12057-10c7-45f3-bb34-427c0f3b6ad5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_02d12057-10c7-45f3-bb34-427c0f3b6ad5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_02d12057-10c7-45f3-bb34-427c0f3b6ad5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_02d12057-10c7-45f3-bb34-427c0f3b6ad5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_02d12057-10c7-45f3-bb34-427c0f3b6ad5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_0c145222-1689-44a2-924e-f5c5d9eb4807" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_0c145222-1689-44a2-924e-f5c5d9eb4807" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_c96d1ff5-fa2f-48ba-9a08-76c7b9cb03d7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:to="loc_us-gaap_CustomerRelationshipsMember_c96d1ff5-fa2f-48ba-9a08-76c7b9cb03d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_8af03e2d-6a76-497c-b7e7-e462286c76a3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TrademarksAndTradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_8af03e2d-6a76-497c-b7e7-e462286c76a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_4474aeac-d53a-41d1-9a92-3d7b5f476444" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_4474aeac-d53a-41d1-9a92-3d7b5f476444" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_363439c6-2113-4579-8886-d366087402b8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ce0286ef-e19c-43db-9e22-7b1e5a19dd90" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_363439c6-2113-4579-8886-d366087402b8" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsSpotXandSpringServeUnauditedProFormaInformationDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#BusinessCombinationsSpotXandSpringServeUnauditedProFormaInformationDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsSpotXandSpringServeUnauditedProFormaInformationDetails" xlink:type="extended" id="ib74b1b821ae74c37b8bc51f48f3cd933_BusinessCombinationsSpotXandSpringServeUnauditedProFormaInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_2dfba392-cac9-403e-901a-865f192ae051" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_52be3eac-8987-471f-b5ef-edc4411f464a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2dfba392-cac9-403e-901a-865f192ae051" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_52be3eac-8987-471f-b5ef-edc4411f464a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_89507e7f-d0f1-4406-ae48-043219a2123a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2dfba392-cac9-403e-901a-865f192ae051" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_89507e7f-d0f1-4406-ae48-043219a2123a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7d8d0baf-bf64-4b22-b0dd-a676c93b0c7b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2dfba392-cac9-403e-901a-865f192ae051" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7d8d0baf-bf64-4b22-b0dd-a676c93b0c7b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7d8d0baf-bf64-4b22-b0dd-a676c93b0c7b" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_753b572a-7b95-4c6b-8a4f-c61369f6c83f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1887ba5b-c2a2-434c-a871-4e2a0a69fb2a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_753b572a-7b95-4c6b-8a4f-c61369f6c83f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXIncAndServeMotionIncMember_281024b0-71c5-46c3-9b34-16610c74cec2" xlink:href="mgni-20220630.xsd#mgni_SpotXIncAndServeMotionIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_753b572a-7b95-4c6b-8a4f-c61369f6c83f" xlink:to="loc_mgni_SpotXIncAndServeMotionIncMember_281024b0-71c5-46c3-9b34-16610c74cec2" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsMergerandRestructuringCostActivityDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#MergerAcquisitionandRestructuringCostsMergerandRestructuringCostActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsMergerandRestructuringCostActivityDetails" xlink:type="extended" id="ief13f33c24ec41be9be00c42844be5bf_MergerAcquisitionandRestructuringCostsMergerandRestructuringCostActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_c47c5f14-43a4-4d43-9f92-7ce9960582cd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_c47c5f14-43a4-4d43-9f92-7ce9960582cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_c5d90e1a-56cb-4f75-a11e-6a5fc8ae6868" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_us-gaap_RestructuringCharges_c5d90e1a-56cb-4f75-a11e-6a5fc8ae6868" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_276481c2-5793-4bb7-9c87-de92a01608eb" xlink:href="mgni-20220630.xsd#mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_276481c2-5793-4bb7-9c87-de92a01608eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentsSettledWithoutCash_d37b9851-f39b-4202-b958-43af8e15648c" xlink:href="mgni-20220630.xsd#mgni_RestructuringImpairmentsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_mgni_RestructuringImpairmentsSettledWithoutCash_d37b9851-f39b-4202-b958-43af8e15648c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_a8bb0fb9-6a0b-4599-9010-bf220ca1115f" xlink:href="mgni-20220630.xsd#mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_a8bb0fb9-6a0b-4599-9010-bf220ca1115f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_414c2cec-309f-4aac-9048-fc91e279f606" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_dafb4a6c-957b-401f-8ad6-78210d10c753" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_414c2cec-309f-4aac-9048-fc91e279f606" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_fd09fdbf-1219-495c-95f6-8bb2c28b12e6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_414c2cec-309f-4aac-9048-fc91e279f606" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_fd09fdbf-1219-495c-95f6-8bb2c28b12e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_fd09fdbf-1219-495c-95f6-8bb2c28b12e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_fd09fdbf-1219-495c-95f6-8bb2c28b12e6" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_fd09fdbf-1219-495c-95f6-8bb2c28b12e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_8f704db4-7e76-45d7-a66a-5ffc4c17dca9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_fd09fdbf-1219-495c-95f6-8bb2c28b12e6" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_8f704db4-7e76-45d7-a66a-5ffc4c17dca9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember_f0eae753-6c40-4a53-aaa3-13d2d74f4759" xlink:href="mgni-20220630.xsd#mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_8f704db4-7e76-45d7-a66a-5ffc4c17dca9" xlink:to="loc_mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember_f0eae753-6c40-4a53-aaa3-13d2d74f4759" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LeaseRelatedLossContractsMember_4020d7a5-a192-4891-80f9-679eb7bb9553" xlink:href="mgni-20220630.xsd#mgni_LeaseRelatedLossContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_8f704db4-7e76-45d7-a66a-5ffc4c17dca9" xlink:to="loc_mgni_LeaseRelatedLossContractsMember_4020d7a5-a192-4891-80f9-679eb7bb9553" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#MergerAcquisitionandRestructuringCostsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsNarrativeDetails" xlink:type="extended" id="i7d307cc3253c438e8fbd643110da34f6_MergerAcquisitionandRestructuringCostsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_f4a6ae35-afa3-4a0e-9d2e-736996e1833e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_48c0e607-c762-4a12-8157-5660acefcb5d" xlink:href="mgni-20220630.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_f4a6ae35-afa3-4a0e-9d2e-736996e1833e" xlink:to="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_48c0e607-c762-4a12-8157-5660acefcb5d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_f4963cb2-c8a4-4eda-bb32-e833834f15cd" xlink:href="mgni-20220630.xsd#mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_f4a6ae35-afa3-4a0e-9d2e-736996e1833e" xlink:to="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_f4963cb2-c8a4-4eda-bb32-e833834f15cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_d2f0ada8-f4a7-4324-bd14-7297e365466e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_f4a6ae35-afa3-4a0e-9d2e-736996e1833e" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_d2f0ada8-f4a7-4324-bd14-7297e365466e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_b7ef979d-0c1e-44b8-b03b-cf24f24075ed" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_d2f0ada8-f4a7-4324-bd14-7297e365466e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_b7ef979d-0c1e-44b8-b03b-cf24f24075ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b7ef979d-0c1e-44b8-b03b-cf24f24075ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_b7ef979d-0c1e-44b8-b03b-cf24f24075ed" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b7ef979d-0c1e-44b8-b03b-cf24f24075ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_4b67b32c-19c5-41c1-984f-556f426773bc" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_b7ef979d-0c1e-44b8-b03b-cf24f24075ed" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_4b67b32c-19c5-41c1-984f-556f426773bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXAcquisitionSpringServeAcquisitionAndTelariaMergerMember_ccafeff8-0750-4258-94ee-519e46494f0a" xlink:href="mgni-20220630.xsd#mgni_SpotXAcquisitionSpringServeAcquisitionAndTelariaMergerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_4b67b32c-19c5-41c1-984f-556f426773bc" xlink:to="loc_mgni_SpotXAcquisitionSpringServeAcquisitionAndTelariaMergerMember_ccafeff8-0750-4258-94ee-519e46494f0a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsAccruedMergerandRestructuringCostActivityDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#MergerAcquisitionandRestructuringCostsAccruedMergerandRestructuringCostActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsAccruedMergerandRestructuringCostActivityDetails" xlink:type="extended" id="id727c9c655254d2e98803bba7451c1ce_MergerAcquisitionandRestructuringCostsAccruedMergerandRestructuringCostActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_ded94f4a-c756-40ed-ad60-a1b17039346d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_ded94f4a-c756-40ed-ad60-a1b17039346d" xlink:to="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_891547fc-9fe5-4414-9581-febe81bb65bd" xlink:href="mgni-20220630.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_891547fc-9fe5-4414-9581-febe81bb65bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash_9d02bb47-68c7-4c43-96f1-2dc9ba6a701e" xlink:href="mgni-20220630.xsd#mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash_9d02bb47-68c7-4c43-96f1-2dc9ba6a701e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentCosts_2560119e-bed5-45b4-8ec3-eadd1797b054" xlink:href="mgni-20220630.xsd#mgni_RestructuringImpairmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_RestructuringImpairmentCosts_2560119e-bed5-45b4-8ec3-eadd1797b054" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PaymentsForRestructuringAndMergerRelatedCosts_5186e827-154f-4757-a1d3-b9dccd771b70" xlink:href="mgni-20220630.xsd#mgni_PaymentsForRestructuringAndMergerRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_PaymentsForRestructuringAndMergerRelatedCosts_5186e827-154f-4757-a1d3-b9dccd771b70" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentsSettledWithoutCash_428ea316-df05-45a0-983d-915c8017668c" xlink:href="mgni-20220630.xsd#mgni_RestructuringImpairmentsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_RestructuringImpairmentsSettledWithoutCash_428ea316-df05-45a0-983d-915c8017668c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_90b19ffb-47c2-4fed-92e5-320fd96ac29e" xlink:href="mgni-20220630.xsd#mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_c30d1131-a95b-4cea-9ebf-56f19ddad1e6" xlink:to="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_90b19ffb-47c2-4fed-92e5-320fd96ac29e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_de6db6af-d990-4840-a11e-ce52999bc935" xlink:href="mgni-20220630.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_68b1f191-a8af-415f-a1fa-9ce151378d47" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_ded94f4a-c756-40ed-ad60-a1b17039346d" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_68b1f191-a8af-415f-a1fa-9ce151378d47" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_5da87b2c-5965-4c9c-956f-1bb995aade26" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_68b1f191-a8af-415f-a1fa-9ce151378d47" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_5da87b2c-5965-4c9c-956f-1bb995aade26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5da87b2c-5965-4c9c-956f-1bb995aade26_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5da87b2c-5965-4c9c-956f-1bb995aade26" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5da87b2c-5965-4c9c-956f-1bb995aade26_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42a8a8a2-f1c3-4fe9-aef9-c050d366c0d3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5da87b2c-5965-4c9c-956f-1bb995aade26" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42a8a8a2-f1c3-4fe9-aef9-c050d366c0d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXAndTelariaMember_bebb4b04-610e-4d53-aea9-7ad26ad8053b" xlink:href="mgni-20220630.xsd#mgni_SpotXAndTelariaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42a8a8a2-f1c3-4fe9-aef9-c050d366c0d3" xlink:to="loc_mgni_SpotXAndTelariaMember_bebb4b04-610e-4d53-aea9-7ad26ad8053b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended" id="i4feb65f788eb437594dc5622fb81b8ef_StockBasedCompensationNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_3b583845-9abe-4b33-b75d-f2316123dd34" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_3b583845-9abe-4b33-b75d-f2316123dd34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_fa761d0b-521b-40e5-8cc7-889492d4d158" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_fa761d0b-521b-40e5-8cc7-889492d4d158" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_62e8b3bb-c761-46f9-acfb-8a86ed8d596b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_62e8b3bb-c761-46f9-acfb-8a86ed8d596b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_081f85c4-66f2-43b1-a818-83124dfe3d9e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_081f85c4-66f2-43b1-a818-83124dfe3d9e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EvergreenAnnualIncrease_ea5ca39f-fa8c-4687-9998-0099a0dbafd9" xlink:href="mgni-20220630.xsd#mgni_EvergreenAnnualIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_mgni_EvergreenAnnualIncrease_ea5ca39f-fa8c-4687-9998-0099a0dbafd9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ad7e30a5-4999-4a35-bbfe-f5c82192f45e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_09b3e3d2-8626-46ab-8073-fb7d2aa2bb85" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ad7e30a5-4999-4a35-bbfe-f5c82192f45e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_72ed7f1f-a855-4bba-9d78-e5c016d6931f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ad7e30a5-4999-4a35-bbfe-f5c82192f45e" xlink:to="loc_us-gaap_AwardTypeAxis_72ed7f1f-a855-4bba-9d78-e5c016d6931f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_72ed7f1f-a855-4bba-9d78-e5c016d6931f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_72ed7f1f-a855-4bba-9d78-e5c016d6931f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_72ed7f1f-a855-4bba-9d78-e5c016d6931f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f516d36-ee17-497b-a4b2-37bdc7ca6b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_72ed7f1f-a855-4bba-9d78-e5c016d6931f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f516d36-ee17-497b-a4b2-37bdc7ca6b8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_f360a742-4321-4476-9b7a-1a79f2caaeda" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f516d36-ee17-497b-a4b2-37bdc7ca6b8f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_f360a742-4321-4476-9b7a-1a79f2caaeda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestrictedStockUnitsAndRestrictedStockAwardsMember_6554a724-c399-4dd3-a556-331c6f57b410" xlink:href="mgni-20220630.xsd#mgni_RestrictedStockUnitsAndRestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f516d36-ee17-497b-a4b2-37bdc7ca6b8f" xlink:to="loc_mgni_RestrictedStockUnitsAndRestrictedStockAwardsMember_6554a724-c399-4dd3-a556-331c6f57b410" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_a677bcad-d132-4e17-ac32-c54636f0d1bb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_RestrictedStockUnitsAndRestrictedStockAwardsMember_6554a724-c399-4dd3-a556-331c6f57b410" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_a677bcad-d132-4e17-ac32-c54636f0d1bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_6259b1e5-56d1-402c-9874-fa30ae745176" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ad7e30a5-4999-4a35-bbfe-f5c82192f45e" xlink:to="loc_us-gaap_VestingAxis_6259b1e5-56d1-402c-9874-fa30ae745176" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_6259b1e5-56d1-402c-9874-fa30ae745176_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_6259b1e5-56d1-402c-9874-fa30ae745176" xlink:to="loc_us-gaap_VestingDomain_6259b1e5-56d1-402c-9874-fa30ae745176_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_b9cfa0f9-9fe1-4fc0-9b49-18fdaccc973f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_6259b1e5-56d1-402c-9874-fa30ae745176" xlink:to="loc_us-gaap_VestingDomain_b9cfa0f9-9fe1-4fc0-9b49-18fdaccc973f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ca0fe4b0-3743-4cd0-9f3f-8a485cb299a3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_b9cfa0f9-9fe1-4fc0-9b49-18fdaccc973f" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ca0fe4b0-3743-4cd0-9f3f-8a485cb299a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_e49ac298-3fd5-4423-a430-9a9c8514682e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_b9cfa0f9-9fe1-4fc0-9b49-18fdaccc973f" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_e49ac298-3fd5-4423-a430-9a9c8514682e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationStockOptionsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationStockOptionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationStockOptionsNarrativeDetails" xlink:type="extended" id="i4a99985ec7d248568fc0ba8132504eec_StockBasedCompensationStockOptionsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_4e725410-1432-4ef9-a9cc-48764ef0ff26" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_4e725410-1432-4ef9-a9cc-48764ef0ff26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_80ce6515-ddaf-4450-90f8-effa6342d786" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_80ce6515-ddaf-4450-90f8-effa6342d786" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_a9cdaf53-d269-416f-b0b1-9fb64a90f0ff" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_a9cdaf53-d269-416f-b0b1-9fb64a90f0ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_0e06938d-afc6-4b01-b988-70a602e485e0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_0e06938d-afc6-4b01-b988-70a602e485e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_382a36ae-55c0-4ff3-9bfa-3e4abe9f6a90" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_382a36ae-55c0-4ff3-9bfa-3e4abe9f6a90" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74d02fa7-4b6d-4e2c-860b-5b7d1888c9eb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_87a4f132-a71f-4958-8040-c53063914fa3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74d02fa7-4b6d-4e2c-860b-5b7d1888c9eb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_375ecc1b-0fa1-4255-abc6-0eb698dd43b1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74d02fa7-4b6d-4e2c-860b-5b7d1888c9eb" xlink:to="loc_us-gaap_AwardTypeAxis_375ecc1b-0fa1-4255-abc6-0eb698dd43b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_375ecc1b-0fa1-4255-abc6-0eb698dd43b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_375ecc1b-0fa1-4255-abc6-0eb698dd43b1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_375ecc1b-0fa1-4255-abc6-0eb698dd43b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3dc328ed-b195-4625-87b2-0aadd521978e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_375ecc1b-0fa1-4255-abc6-0eb698dd43b1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3dc328ed-b195-4625-87b2-0aadd521978e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_7050b5fd-ec50-4dd9-9083-ee6029412f71" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3dc328ed-b195-4625-87b2-0aadd521978e" xlink:to="loc_us-gaap_EmployeeStockOptionMember_7050b5fd-ec50-4dd9-9083-ee6029412f71" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationValuationAssumptionsDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationValuationAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationValuationAssumptionsDetails" xlink:type="extended" id="icca5dd93217d49da8965fb198fb20502_StockBasedCompensationValuationAssumptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_398b4f06-0fbb-4e76-9920-36491fcc25c3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_398b4f06-0fbb-4e76-9920-36491fcc25c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_e6c8d35b-7dc9-40be-8c78-5337b77ed78d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_e6c8d35b-7dc9-40be-8c78-5337b77ed78d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_8588190a-a251-46fd-abf1-7063f6ed0d44" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_8588190a-a251-46fd-abf1-7063f6ed0d44" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_9a4ffc92-4fe4-48f6-bbb7-8d1dbd521b54" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_9a4ffc92-4fe4-48f6-bbb7-8d1dbd521b54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4b29d7ba-31d2-46d3-b24d-867498caa047" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1ce05c32-74ab-48e2-8bd3-da082afbf4b3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4b29d7ba-31d2-46d3-b24d-867498caa047" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_c1cb04e4-5a86-4230-9d5b-12792da09f14" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4b29d7ba-31d2-46d3-b24d-867498caa047" xlink:to="loc_us-gaap_AwardTypeAxis_c1cb04e4-5a86-4230-9d5b-12792da09f14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1cb04e4-5a86-4230-9d5b-12792da09f14_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_c1cb04e4-5a86-4230-9d5b-12792da09f14" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1cb04e4-5a86-4230-9d5b-12792da09f14_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c7c0c4d7-2b51-43a0-849e-fb8dd2e52910" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_c1cb04e4-5a86-4230-9d5b-12792da09f14" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c7c0c4d7-2b51-43a0-849e-fb8dd2e52910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_407ad77d-3bb4-4011-83cd-dc316b6ad910" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c7c0c4d7-2b51-43a0-849e-fb8dd2e52910" xlink:to="loc_us-gaap_EmployeeStockOptionMember_407ad77d-3bb4-4011-83cd-dc316b6ad910" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsActivityDetails" xlink:type="extended" id="i719ffd53fb344235966f6972334adef0_StockBasedCompensationRestrictedStockUnitsActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_681291bb-2293-403b-9f42-5e12650d6318" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_681291bb-2293-403b-9f42-5e12650d6318" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b772eebd-491e-4942-9aff-9e51370fd928" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b772eebd-491e-4942-9aff-9e51370fd928" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3ed8cfb1-95f3-4305-beef-3fd31fe77b23" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3ed8cfb1-95f3-4305-beef-3fd31fe77b23" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f732c8e5-4960-42e6-bd5d-3f840dd5ae0b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_b622caec-010f-4c55-a961-f914a5411969" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f732c8e5-4960-42e6-bd5d-3f840dd5ae0b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_61de947b-42a0-45f9-bb75-7ba5a633566a" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_48f896a2-b779-489b-95fd-29e6fa27189f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_48f896a2-b779-489b-95fd-29e6fa27189f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_3a41acc0-2726-4169-a4b4-13d27ba6c506" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_3a41acc0-2726-4169-a4b4-13d27ba6c506" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8ea89215-2f0c-4120-b5f6-f5d8b146b79f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8ea89215-2f0c-4120-b5f6-f5d8b146b79f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_04adf21a-b3a6-4068-ace0-9fdae0fef694" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_04adf21a-b3a6-4068-ace0-9fdae0fef694" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_60bbe1ad-e8fc-43bb-9c81-675897dc974e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_20e760e5-ca19-4d27-b110-18d35da0a1ed" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_60bbe1ad-e8fc-43bb-9c81-675897dc974e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_3231f745-6000-4aa5-99a6-406997fab9c8" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_35b2b9b3-bd08-4a69-b7b9-bbf080fa3099" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_35b2b9b3-bd08-4a69-b7b9-bbf080fa3099" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedButDeferredNumber_13ec9516-4024-4caf-81a4-4402dfca7fd6" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedButDeferredNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedButDeferredNumber_13ec9516-4024-4caf-81a4-4402dfca7fd6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_105e8a0d-f258-45db-a4c9-c56dfb860cc3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d66035de-cc71-4415-994a-4b2095403730" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_105e8a0d-f258-45db-a4c9-c56dfb860cc3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_77b7830e-e34d-4e0e-8c44-ad3453faf65c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_105e8a0d-f258-45db-a4c9-c56dfb860cc3" xlink:to="loc_us-gaap_AwardTypeAxis_77b7830e-e34d-4e0e-8c44-ad3453faf65c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77b7830e-e34d-4e0e-8c44-ad3453faf65c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_77b7830e-e34d-4e0e-8c44-ad3453faf65c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77b7830e-e34d-4e0e-8c44-ad3453faf65c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cb4c9810-c0d7-4c94-ab73-a6cc386b5d38" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_77b7830e-e34d-4e0e-8c44-ad3453faf65c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cb4c9810-c0d7-4c94-ab73-a6cc386b5d38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_b27d104a-767c-4db0-b501-cbdba3377025" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cb4c9810-c0d7-4c94-ab73-a6cc386b5d38" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_b27d104a-767c-4db0-b501-cbdba3377025" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationRestrictedStockUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsNarrativeDetails" xlink:type="extended" id="i619b88b92bb049bf83ddb35fe361616d_StockBasedCompensationRestrictedStockUnitsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_12515aa9-3851-42d5-9916-d630599e51aa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_12515aa9-3851-42d5-9916-d630599e51aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_04d5c8c9-6a61-426b-99b1-a3fe5eecd11d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_04d5c8c9-6a61-426b-99b1-a3fe5eecd11d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_a32a7a61-ef6e-4ed1-b0f6-26702d1ac085" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_a32a7a61-ef6e-4ed1-b0f6-26702d1ac085" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_9a610b02-b673-401f-b7ff-c98eba9c316c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_9a610b02-b673-401f-b7ff-c98eba9c316c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_802f117a-70de-411e-8f1e-485ba90d0e92" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_802f117a-70de-411e-8f1e-485ba90d0e92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_db3918e4-b5bd-4e23-abc7-3737e1531923" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_083b96ac-2f0e-4704-bbc5-cca31df13d2a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_db3918e4-b5bd-4e23-abc7-3737e1531923" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_a9075375-633c-4d22-b3f5-fa81324fa9aa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_db3918e4-b5bd-4e23-abc7-3737e1531923" xlink:to="loc_us-gaap_AwardTypeAxis_a9075375-633c-4d22-b3f5-fa81324fa9aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a9075375-633c-4d22-b3f5-fa81324fa9aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_a9075375-633c-4d22-b3f5-fa81324fa9aa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a9075375-633c-4d22-b3f5-fa81324fa9aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_abd1e464-168a-4fbd-a355-320ff2fe9d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_a9075375-633c-4d22-b3f5-fa81324fa9aa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_abd1e464-168a-4fbd-a355-320ff2fe9d4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_e939748d-347c-4116-8388-ed782f346157" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_abd1e464-168a-4fbd-a355-320ff2fe9d4e" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_e939748d-347c-4116-8388-ed782f346157" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationPerformanceStockUnitsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationPerformanceStockUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationPerformanceStockUnitsNarrativeDetails" xlink:type="extended" id="i788f79f604654d798b276d807fcaa159_StockBasedCompensationPerformanceStockUnitsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b9b630f7-b57b-49be-9373-5649506f4e49" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b9b630f7-b57b-49be-9373-5649506f4e49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NumberOfTranches_4702ea90-839b-42a8-8d37-e3d9bd5cf6af" xlink:href="mgni-20220630.xsd#mgni_NumberOfTranches"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_mgni_NumberOfTranches_4702ea90-839b-42a8-8d37-e3d9bd5cf6af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_1d28c1eb-4dec-46ae-9967-b3798c051827" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_1d28c1eb-4dec-46ae-9967-b3798c051827" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_38f5ddb9-dbb8-4472-94fd-706df710fb7a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_38f5ddb9-dbb8-4472-94fd-706df710fb7a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue_c5b81f03-c800-4f75-80a2-2fdf139e4c41" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue_c5b81f03-c800-4f75-80a2-2fdf139e4c41" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a1ac6cd5-b717-4bbd-bb73-ce272dfea58a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_a1ac6cd5-b717-4bbd-bb73-ce272dfea58a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_c9299295-6faa-4d1e-9520-1271bc145bed" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_c9299295-6faa-4d1e-9520-1271bc145bed" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_d705c971-7ef6-45e2-a4b9-e3f135e1586a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_d705c971-7ef6-45e2-a4b9-e3f135e1586a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_9b52f5b3-7d35-4f81-8ed3-b88b2b1d84a9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_9b52f5b3-7d35-4f81-8ed3-b88b2b1d84a9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays_5e5435fc-74a1-4ca9-a317-601f4ce64350" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays_5e5435fc-74a1-4ca9-a317-601f4ce64350" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger_865221b3-fb14-4491-adc8-4a48b9435fe6" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger_865221b3-fb14-4491-adc8-4a48b9435fe6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_29a093cb-7eee-4b46-a7dc-6b4f026f8724" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_29a093cb-7eee-4b46-a7dc-6b4f026f8724" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_4dce5299-159e-49ea-8923-001fc3790007" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_4dce5299-159e-49ea-8923-001fc3790007" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_b86dbb49-c9f5-4a84-9e1c-159d1591d885" xlink:href="mgni-20220630.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_b86dbb49-c9f5-4a84-9e1c-159d1591d885" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_16ed113b-ccca-46ea-86a5-112671a5a5da" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_23e48963-b5c3-4128-8ee9-de21ae03ab42" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:to="loc_us-gaap_AwardTypeAxis_23e48963-b5c3-4128-8ee9-de21ae03ab42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_23e48963-b5c3-4128-8ee9-de21ae03ab42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_23e48963-b5c3-4128-8ee9-de21ae03ab42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_23e48963-b5c3-4128-8ee9-de21ae03ab42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3a2bd12a-be99-4030-9e90-16555f1df47c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_23e48963-b5c3-4128-8ee9-de21ae03ab42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3a2bd12a-be99-4030-9e90-16555f1df47c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_90dc7dba-672e-45c1-a852-59fd2ca1a82a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3a2bd12a-be99-4030-9e90-16555f1df47c" xlink:to="loc_us-gaap_PerformanceSharesMember_90dc7dba-672e-45c1-a852-59fd2ca1a82a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedApril2020Member_362977c2-3383-464e-848e-057fdf7ebce6" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedApril2020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_90dc7dba-672e-45c1-a852-59fd2ca1a82a" xlink:to="loc_mgni_PerformanceSharesGrantedApril2020Member_362977c2-3383-464e-848e-057fdf7ebce6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedApril2021Member_0adf264c-0585-4837-987c-d9844ecf3886" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedApril2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_90dc7dba-672e-45c1-a852-59fd2ca1a82a" xlink:to="loc_mgni_PerformanceSharesGrantedApril2021Member_0adf264c-0585-4837-987c-d9844ecf3886" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedFebruary2022Member_a058b421-ddb6-4822-90e3-317bcecd225b" xlink:href="mgni-20220630.xsd#mgni_PerformanceSharesGrantedFebruary2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_90dc7dba-672e-45c1-a852-59fd2ca1a82a" xlink:to="loc_mgni_PerformanceSharesGrantedFebruary2022Member_a058b421-ddb6-4822-90e3-317bcecd225b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_f585cdd0-246c-4078-973f-fa3007f1051a" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:to="loc_srt_RangeAxis_f585cdd0-246c-4078-973f-fa3007f1051a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_f585cdd0-246c-4078-973f-fa3007f1051a_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_f585cdd0-246c-4078-973f-fa3007f1051a" xlink:to="loc_srt_RangeMember_f585cdd0-246c-4078-973f-fa3007f1051a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_1218de47-b4b3-4dc6-9579-d372602a66a2" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_f585cdd0-246c-4078-973f-fa3007f1051a" xlink:to="loc_srt_RangeMember_1218de47-b4b3-4dc6-9579-d372602a66a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_8d0bfe71-f8c1-4501-ac82-8bb72e2d4327" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_1218de47-b4b3-4dc6-9579-d372602a66a2" xlink:to="loc_srt_MinimumMember_8d0bfe71-f8c1-4501-ac82-8bb72e2d4327" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_59bba409-9708-4e0f-9348-1b15b5b6df1c" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_1218de47-b4b3-4dc6-9579-d372602a66a2" xlink:to="loc_srt_MaximumMember_59bba409-9708-4e0f-9348-1b15b5b6df1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_ad9c06d4-f30f-42fc-9818-4f9ee66c719c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:to="loc_us-gaap_VestingAxis_ad9c06d4-f30f-42fc-9818-4f9ee66c719c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_ad9c06d4-f30f-42fc-9818-4f9ee66c719c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_ad9c06d4-f30f-42fc-9818-4f9ee66c719c" xlink:to="loc_us-gaap_VestingDomain_ad9c06d4-f30f-42fc-9818-4f9ee66c719c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_ad9c06d4-f30f-42fc-9818-4f9ee66c719c" xlink:to="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SharebasedPaymentArrangementTrancheFourMember_609895ff-35a8-49eb-a7fe-b3714b34e7ea" xlink:href="mgni-20220630.xsd#mgni_SharebasedPaymentArrangementTrancheFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:to="loc_mgni_SharebasedPaymentArrangementTrancheFourMember_609895ff-35a8-49eb-a7fe-b3714b34e7ea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_3e58fc44-a849-4cd1-83c0-33e4b60bea82" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_3e58fc44-a849-4cd1-83c0-33e4b60bea82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_34e38763-bc18-4323-a78f-489aa0f6ef20" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_34e38763-bc18-4323-a78f-489aa0f6ef20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_ba0c1990-32f3-47ba-b216-842de1b9f68c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_358683c1-c14a-4222-8359-a221f2578d00" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_ba0c1990-32f3-47ba-b216-842de1b9f68c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_170e8a86-5ec9-4c51-ab71-88e2aa7fddae" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_12f59cf9-729e-4d56-add4-db0a07eea77a" xlink:to="loc_srt_TitleOfIndividualAxis_170e8a86-5ec9-4c51-ab71-88e2aa7fddae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_170e8a86-5ec9-4c51-ab71-88e2aa7fddae_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_170e8a86-5ec9-4c51-ab71-88e2aa7fddae" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_170e8a86-5ec9-4c51-ab71-88e2aa7fddae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_4257c2f9-037d-4dde-b0a3-cf8b93cca416" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_170e8a86-5ec9-4c51-ab71-88e2aa7fddae" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_4257c2f9-037d-4dde-b0a3-cf8b93cca416" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_44406ebd-d32b-452a-87fa-57aa0428e9a0" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_4257c2f9-037d-4dde-b0a3-cf8b93cca416" xlink:to="loc_srt_ChiefExecutiveOfficerMember_44406ebd-d32b-452a-87fa-57aa0428e9a0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails" xlink:type="extended" id="id3353ff5d1fb4d0e89b8cb8e81ca7dc2_StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_de09fcb6-32fb-4e68-91e0-938ecab2efd4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_de09fcb6-32fb-4e68-91e0-938ecab2efd4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SharebasedCompensationArrangementbySharebasedPaymentAwardOfferingPeriod_deef688e-d74a-46c3-8433-c0ff4e44cfba" xlink:href="mgni-20220630.xsd#mgni_SharebasedCompensationArrangementbySharebasedPaymentAwardOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_mgni_SharebasedCompensationArrangementbySharebasedPaymentAwardOfferingPeriod_deef688e-d74a-46c3-8433-c0ff4e44cfba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_4270e7c8-8376-4bf5-a1fb-c7d81277c2e1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_4270e7c8-8376-4bf5-a1fb-c7d81277c2e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7c0112bb-ccaf-400e-bc35-2c024287b9a8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7c0112bb-ccaf-400e-bc35-2c024287b9a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EvergreenAnnualIncrease_6af78939-f273-4c4c-811f-d9588ef75f49" xlink:href="mgni-20220630.xsd#mgni_EvergreenAnnualIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_mgni_EvergreenAnnualIncrease_6af78939-f273-4c4c-811f-d9588ef75f49" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_96f7e6d1-8aa3-44bf-8a8d-b16f3e6c40bf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1d7f85aa-f059-4744-8ebe-67a8e4d63e6c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_96f7e6d1-8aa3-44bf-8a8d-b16f3e6c40bf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_907f1756-d272-4c26-8bf8-1b345c4c0a28" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_96f7e6d1-8aa3-44bf-8a8d-b16f3e6c40bf" xlink:to="loc_us-gaap_PlanNameAxis_907f1756-d272-4c26-8bf8-1b345c4c0a28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_907f1756-d272-4c26-8bf8-1b345c4c0a28_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_907f1756-d272-4c26-8bf8-1b345c4c0a28" xlink:to="loc_us-gaap_PlanNameDomain_907f1756-d272-4c26-8bf8-1b345c4c0a28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_ec8f01fe-7a3b-45e7-b5ff-94e3e8f8d64e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_907f1756-d272-4c26-8bf8-1b345c4c0a28" xlink:to="loc_us-gaap_PlanNameDomain_ec8f01fe-7a3b-45e7-b5ff-94e3e8f8d64e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_A2014EmployeeStockPurchasePlanMember_e6d0dadb-0a7f-4c3d-b988-4e65827457f2" xlink:href="mgni-20220630.xsd#mgni_A2014EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ec8f01fe-7a3b-45e7-b5ff-94e3e8f8d64e" xlink:to="loc_mgni_A2014EmployeeStockPurchasePlanMember_e6d0dadb-0a7f-4c3d-b988-4e65827457f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_f9cdc52d-8d04-48cf-a799-8392f55d093e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_96f7e6d1-8aa3-44bf-8a8d-b16f3e6c40bf" xlink:to="loc_us-gaap_AwardTypeAxis_f9cdc52d-8d04-48cf-a799-8392f55d093e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f9cdc52d-8d04-48cf-a799-8392f55d093e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_f9cdc52d-8d04-48cf-a799-8392f55d093e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f9cdc52d-8d04-48cf-a799-8392f55d093e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_86ea57af-0379-4a9e-bbad-7f8a4e04b3c5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_f9cdc52d-8d04-48cf-a799-8392f55d093e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_86ea57af-0379-4a9e-bbad-7f8a4e04b3c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_7095f3a7-8d3c-43a3-9705-3f44d5e6be6a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_86ea57af-0379-4a9e-bbad-7f8a4e04b3c5" xlink:to="loc_us-gaap_EmployeeStockMember_7095f3a7-8d3c-43a3-9705-3f44d5e6be6a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#StockBasedCompensationStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationStockBasedCompensationExpenseDetails" xlink:type="extended" id="ib23b084db9e947b58586a979a8eaa192_StockBasedCompensationStockBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_bf1a76e7-8e16-4ffe-9314-da08a0fcfdc9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_9dd7ef42-c7e8-4aa6-b04d-ef479beee0d3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_bf1a76e7-8e16-4ffe-9314-da08a0fcfdc9" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_9dd7ef42-c7e8-4aa6-b04d-ef479beee0d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_093a981c-1840-49bf-9844-dfd95e1b0915" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_bf1a76e7-8e16-4ffe-9314-da08a0fcfdc9" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_093a981c-1840-49bf-9844-dfd95e1b0915" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_8233fc88-c6f5-4e54-87f1-046cccfdd742" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_093a981c-1840-49bf-9844-dfd95e1b0915" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_8233fc88-c6f5-4e54-87f1-046cccfdd742" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_8233fc88-c6f5-4e54-87f1-046cccfdd742_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8233fc88-c6f5-4e54-87f1-046cccfdd742" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_8233fc88-c6f5-4e54-87f1-046cccfdd742_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8233fc88-c6f5-4e54-87f1-046cccfdd742" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_39daabe4-f4b5-4a08-bea3-bc1507cb4a2d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:to="loc_us-gaap_CostOfSalesMember_39daabe4-f4b5-4a08-bea3-bc1507cb4a2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_f297feeb-c049-4e39-b050-121efb36b5e3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_f297feeb-c049-4e39-b050-121efb36b5e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_3cd50a6e-c6ac-43f3-9fbf-0169052fcca5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_3cd50a6e-c6ac-43f3-9fbf-0169052fcca5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6aa2287e-9a52-4c39-8c71-17e7b3e933ba" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6aa2287e-9a52-4c39-8c71-17e7b3e933ba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_MergerAcquisitionAndRestructuringChargesMember_043fc5de-3d76-4655-b8b7-b3ded6403c00" xlink:href="mgni-20220630.xsd#mgni_MergerAcquisitionAndRestructuringChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2b2c9f49-f5f7-4f87-bbfa-96601096a056" xlink:to="loc_mgni_MergerAcquisitionAndRestructuringChargesMember_043fc5de-3d76-4655-b8b7-b3ded6403c00" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/LeaseObligationsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#LeaseObligationsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/LeaseObligationsNarrativeDetails" xlink:type="extended" id="iaa0cbd6afca140c8b8cc71150c276f89_LeaseObligationsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_da41939a-b26d-41dd-a605-0e5aff51fe84" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_OperatingLeaseExpense_da41939a-b26d-41dd-a605-0e5aff51fe84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermLeaseCost_22596cbc-576a-40e8-824f-4031acd8dbf7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShortTermLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_ShortTermLeaseCost_22596cbc-576a-40e8-824f-4031acd8dbf7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_b7230da7-a5fc-4925-8025-8c71184c2e2d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_VariableLeaseCost_b7230da7-a5fc-4925-8025-8c71184c2e2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubleaseIncome_7e55f071-4027-400c-b35d-0a9219c7d7cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubleaseIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_SubleaseIncome_7e55f071-4027-400c-b35d-0a9219c7d7cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_bd53a45c-5eee-4fdb-8832-9a42d73024a4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_bd53a45c-5eee-4fdb-8832-9a42d73024a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRemainingLeaseTerm_8fcc886e-135f-49bb-b098-afde5ffa54c4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseRemainingLeaseTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_LesseeOperatingLeaseRemainingLeaseTerm_8fcc886e-135f-49bb-b098-afde5ffa54c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_ba91e66d-fc34-42bd-8e9a-a8919f3e8940" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_ba91e66d-fc34-42bd-8e9a-a8919f3e8940" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LesseeOperatingLeaseLeaseNotYetCommencedCommitmentAmount_51047d4a-f14f-4784-b4f4-da01d3100d34" xlink:href="mgni-20220630.xsd#mgni_LesseeOperatingLeaseLeaseNotYetCommencedCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_mgni_LesseeOperatingLeaseLeaseNotYetCommencedCommitmentAmount_51047d4a-f14f-4784-b4f4-da01d3100d34" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_5ec4ac0d-71d3-4190-8df7-0617dc355249" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_5ec4ac0d-71d3-4190-8df7-0617dc355249" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_67c5807f-f53e-4daf-914f-a00c72e4fe44" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_a6ef03a9-1364-45d0-9d87-c0225b5574d7" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_67c5807f-f53e-4daf-914f-a00c72e4fe44" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_67c5807f-f53e-4daf-914f-a00c72e4fe44" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3cd22495-d8a9-4e35-9fc6-db1fdb95cffa" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_fe51c7ca-f6d6-4aa4-ab49-e127369ca9bd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3cd22495-d8a9-4e35-9fc6-db1fdb95cffa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DataCentersForCloudBasedServicesMember_5faecb37-7060-4edf-b6fa-2e48dff8f929" xlink:href="mgni-20220630.xsd#mgni_DataCentersForCloudBasedServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3cd22495-d8a9-4e35-9fc6-db1fdb95cffa" xlink:to="loc_mgni_DataCentersForCloudBasedServicesMember_5faecb37-7060-4edf-b6fa-2e48dff8f929" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CompanyOfficeSanFranciscoCAMember_c809d87f-293d-4e9c-b0f9-83a52dfafadc" xlink:href="mgni-20220630.xsd#mgni_CompanyOfficeSanFranciscoCAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3cd22495-d8a9-4e35-9fc6-db1fdb95cffa" xlink:to="loc_mgni_CompanyOfficeSanFranciscoCAMember_c809d87f-293d-4e9c-b0f9-83a52dfafadc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_67c5807f-f53e-4daf-914f-a00c72e4fe44" xlink:to="loc_srt_RangeAxis_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c" xlink:to="loc_srt_RangeMember_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_febb1bf9-52cf-409d-96e3-e51f6dc8e4e0" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_4192b6cc-c148-4be2-8a4b-d4826b2f4f4c" xlink:to="loc_srt_RangeMember_febb1bf9-52cf-409d-96e3-e51f6dc8e4e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_94f007fb-ee97-45ab-81cd-ad45a87d9bcd" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_febb1bf9-52cf-409d-96e3-e51f6dc8e4e0" xlink:to="loc_srt_MinimumMember_94f007fb-ee97-45ab-81cd-ad45a87d9bcd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_88eadea2-6828-4132-9a45-cdc126ef5454" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_febb1bf9-52cf-409d-96e3-e51f6dc8e4e0" xlink:to="loc_srt_MaximumMember_88eadea2-6828-4132-9a45-cdc126ef5454" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/CommitmentsandContingenciesDetails" xlink:type="extended" id="i21310dd553f641b69c0c173db6b72814_CommitmentsandContingenciesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_40e5fd48-aaaa-4bdf-816c-4fc743ae715c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_f0726aa9-d32b-4067-8625-092504131030" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_40e5fd48-aaaa-4bdf-816c-4fc743ae715c" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_f0726aa9-d32b-4067-8625-092504131030" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_34905781-b2a1-4976-91b3-5f2e2d4d4079" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_40e5fd48-aaaa-4bdf-816c-4fc743ae715c" xlink:to="loc_us-gaap_OtherCommitmentsTable_34905781-b2a1-4976-91b3-5f2e2d4d4079" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_79248210-423b-447c-b779-1f72dde89117" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_34905781-b2a1-4976-91b3-5f2e2d4d4079" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_79248210-423b-447c-b779-1f72dde89117" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_79248210-423b-447c-b779-1f72dde89117_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_79248210-423b-447c-b779-1f72dde89117" xlink:to="loc_us-gaap_LossContingencyNatureDomain_79248210-423b-447c-b779-1f72dde89117_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_3832f497-1102-4ee9-b575-4183223617bf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_79248210-423b-447c-b779-1f72dde89117" xlink:to="loc_us-gaap_LossContingencyNatureDomain_3832f497-1102-4ee9-b575-4183223617bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialStandbyLetterOfCreditMember_bd3a1612-9df1-43b1-9bcd-662709728c42" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinancialStandbyLetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_3832f497-1102-4ee9-b575-4183223617bf" xlink:to="loc_us-gaap_FinancialStandbyLetterOfCreditMember_bd3a1612-9df1-43b1-9bcd-662709728c42" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtScheduleofLongtermDebtDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtScheduleofLongtermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtScheduleofLongtermDebtDetails" xlink:type="extended" id="ifc3b1eac975d440ebd8752333bedf540_DebtScheduleofLongtermDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_4ed63c2d-4956-4854-b0b8-1e0bd5d83173" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_4ed63c2d-4956-4854-b0b8-1e0bd5d83173" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_4e617a28-8db1-4864-9822-8ddc51f8c23b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_4e617a28-8db1-4864-9822-8ddc51f8c23b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_c9a6cb33-0895-48e3-9903-d84a7f9105b9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_LongTermDebt_c9a6cb33-0895-48e3-9903-d84a7f9105b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_a906f66a-8627-4da6-975f-9cabeba95ea0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_LongTermDebtCurrent_a906f66a-8627-4da6-975f-9cabeba95ea0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_9675212c-12a0-4670-b2cc-dbb6e85b9566" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_9675212c-12a0-4670-b2cc-dbb6e85b9566" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_236c945a-e6c9-4fe7-a462-6737f6d63f00" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ca17d9d5-81e8-4ef9-84c2-4d3b5546f1af" xlink:to="loc_us-gaap_DebtInstrumentTable_236c945a-e6c9-4fe7-a462-6737f6d63f00" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_13506326-66aa-4c30-86c4-933e99c20bdb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_236c945a-e6c9-4fe7-a462-6737f6d63f00" xlink:to="loc_us-gaap_DebtInstrumentAxis_13506326-66aa-4c30-86c4-933e99c20bdb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_13506326-66aa-4c30-86c4-933e99c20bdb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_13506326-66aa-4c30-86c4-933e99c20bdb" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_13506326-66aa-4c30-86c4-933e99c20bdb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1d92eca6-7ee2-4479-b5da-3e38f352da91" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_13506326-66aa-4c30-86c4-933e99c20bdb" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1d92eca6-7ee2-4479-b5da-3e38f352da91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_4530d02f-ca9d-4609-b07d-f5931e27bc8c" xlink:href="mgni-20220630.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_1d92eca6-7ee2-4479-b5da-3e38f352da91" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_4530d02f-ca9d-4609-b07d-f5931e27bc8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_912d9c12-d01f-40b4-a08e-2fcf989960ec" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_1d92eca6-7ee2-4479-b5da-3e38f352da91" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_912d9c12-d01f-40b4-a08e-2fcf989960ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_71df87d8-7d11-4203-afa6-0e0bd4f745dd" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_236c945a-e6c9-4fe7-a462-6737f6d63f00" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_71df87d8-7d11-4203-afa6-0e0bd4f745dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_71df87d8-7d11-4203-afa6-0e0bd4f745dd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_71df87d8-7d11-4203-afa6-0e0bd4f745dd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_71df87d8-7d11-4203-afa6-0e0bd4f745dd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_cd11f53f-05af-4a7f-88d1-a99d927a9214" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_71df87d8-7d11-4203-afa6-0e0bd4f745dd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_cd11f53f-05af-4a7f-88d1-a99d927a9214" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_0c3702d9-af12-4936-a7b4-3e6235592424" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_cd11f53f-05af-4a7f-88d1-a99d927a9214" xlink:to="loc_us-gaap_ConvertibleDebtMember_0c3702d9-af12-4936-a7b4-3e6235592424" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_c7c4ae98-37cd-4c7e-8c2d-998bc3e7930b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_cd11f53f-05af-4a7f-88d1-a99d927a9214" xlink:to="loc_us-gaap_SecuredDebtMember_c7c4ae98-37cd-4c7e-8c2d-998bc3e7930b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtMaturitiesofPrincipleAmountofLongTermDebtDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtMaturitiesofPrincipleAmountofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtMaturitiesofPrincipleAmountofLongTermDebtDetails" xlink:type="extended" id="i603a73674c764edb9cb4f826e65c9761_DebtMaturitiesofPrincipleAmountofLongTermDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_b70750db-7902-414c-b60b-5819d6326d61" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_b70750db-7902-414c-b60b-5819d6326d61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_b08b615e-a307-4236-a34b-b450eb87eaf3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_b08b615e-a307-4236-a34b-b450eb87eaf3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_4fddc13d-4556-4c29-8e9e-462d622e581f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_4fddc13d-4556-4c29-8e9e-462d622e581f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_048a6488-c0d3-4c91-9914-daf62ae223d4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_048a6488-c0d3-4c91-9914-daf62ae223d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive_64085bbb-1f75-4fe5-bc74-0dc1efd82ca7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive_64085bbb-1f75-4fe5-bc74-0dc1efd82ca7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive_dd916433-4b5f-49f5-99e5-ba2b935a15c2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive_dd916433-4b5f-49f5-99e5-ba2b935a15c2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_ab5291ad-ecbe-4146-bf23-28054f572590" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_LongTermDebt_ab5291ad-ecbe-4146-bf23-28054f572590" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_f7bdd8a2-1284-4efc-8ab7-c2923a52335c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_371acede-86f6-46b3-8b62-279208e44c44" xlink:to="loc_us-gaap_DebtInstrumentTable_f7bdd8a2-1284-4efc-8ab7-c2923a52335c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_fe568a91-2d94-4831-abcf-6a2f73ed3490" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f7bdd8a2-1284-4efc-8ab7-c2923a52335c" xlink:to="loc_us-gaap_DebtInstrumentAxis_fe568a91-2d94-4831-abcf-6a2f73ed3490" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fe568a91-2d94-4831-abcf-6a2f73ed3490_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_fe568a91-2d94-4831-abcf-6a2f73ed3490" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fe568a91-2d94-4831-abcf-6a2f73ed3490_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c31aa955-ff24-48c9-857c-77c1aa76c2f9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_fe568a91-2d94-4831-abcf-6a2f73ed3490" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c31aa955-ff24-48c9-857c-77c1aa76c2f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_00214746-b736-4d13-9a3b-b2ef8857784e" xlink:href="mgni-20220630.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c31aa955-ff24-48c9-857c-77c1aa76c2f9" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_00214746-b736-4d13-9a3b-b2ef8857784e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_c588ed03-8705-46f9-a9bb-3aac8afc9f67" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c31aa955-ff24-48c9-857c-77c1aa76c2f9" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_c588ed03-8705-46f9-a9bb-3aac8afc9f67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f7bdd8a2-1284-4efc-8ab7-c2923a52335c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ec060212-e0d2-4c2f-8d5f-612e551a84cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_f1fd52b0-11b8-4f65-89a5-bf0efe9f2a7d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ec060212-e0d2-4c2f-8d5f-612e551a84cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_ba9de7ad-dca9-4750-8e53-f5e661614eb8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_ec060212-e0d2-4c2f-8d5f-612e551a84cb" xlink:to="loc_us-gaap_ConvertibleDebtMember_ba9de7ad-dca9-4750-8e53-f5e661614eb8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_6cebcbd1-d24a-4cdc-b79e-d0113e2ce7d6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_ec060212-e0d2-4c2f-8d5f-612e551a84cb" xlink:to="loc_us-gaap_SecuredDebtMember_6cebcbd1-d24a-4cdc-b79e-d0113e2ce7d6" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails" xlink:type="extended" id="if56b8b7aca55417f92932a1376e027b2_DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_aebf7d32-e33d-4793-9d43-beb79e78c51b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_aebf7d32-e33d-4793-9d43-beb79e78c51b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_519a7fe8-cf22-4283-b086-5cef1f52c6e3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_519a7fe8-cf22-4283-b086-5cef1f52c6e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentOverAllotmentOptions_dc6496ff-0a2d-48d8-837b-27555389f006" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentOverAllotmentOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_mgni_DebtInstrumentOverAllotmentOptions_dc6496ff-0a2d-48d8-837b-27555389f006" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromConvertibleDebt_61124dfa-e57a-448a-b368-6fa4b4b5bad5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_ProceedsFromConvertibleDebt_61124dfa-e57a-448a-b368-6fa4b4b5bad5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentsCappedCallsTransactionCosts_3ba48d5e-30c0-4a68-b16e-8fc8ef052479" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentsCappedCallsTransactionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_mgni_DebtInstrumentsCappedCallsTransactionCosts_3ba48d5e-30c0-4a68-b16e-8fc8ef052479" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_0250bb64-6b66-4e5a-90a3-c085d5481e22" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_0250bb64-6b66-4e5a-90a3-c085d5481e22" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_2e7ea5ed-9664-40ab-91ff-221757312329" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_2e7ea5ed-9664-40ab-91ff-221757312329" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_313e533b-8cda-4bbc-8148-22429682f06c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_313e533b-8cda-4bbc-8148-22429682f06c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod_411ea9cc-1373-4099-98b0-fff7ab8be1c2" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod_411ea9cc-1373-4099-98b0-fff7ab8be1c2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault_1e1aa9af-ea61-4b42-8649-30c990eb90ad" xlink:href="mgni-20220630.xsd#mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault_1e1aa9af-ea61-4b42-8649-30c990eb90ad" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_23e65963-3ada-49ca-9e84-3d0f4da9b4b6" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_23e65963-3ada-49ca-9e84-3d0f4da9b4b6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentCappedCallConversionPrice_083782a4-e766-43df-b14d-a33ae1a0d51c" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentCappedCallConversionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_mgni_DebtInstrumentCappedCallConversionPrice_083782a4-e766-43df-b14d-a33ae1a0d51c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_0685b56f-5254-4713-bbaa-ea9166259667" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_0685b56f-5254-4713-bbaa-ea9166259667" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_612ef606-5c71-4612-98da-a44593a7bc91" xlink:to="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_43ba1d70-ae33-447f-8154-52db899d1c90" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:to="loc_us-gaap_DebtInstrumentAxis_43ba1d70-ae33-447f-8154-52db899d1c90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_43ba1d70-ae33-447f-8154-52db899d1c90_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_43ba1d70-ae33-447f-8154-52db899d1c90" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_43ba1d70-ae33-447f-8154-52db899d1c90_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9b7df70e-93a0-421a-a66f-818b6587e4a0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_43ba1d70-ae33-447f-8154-52db899d1c90" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9b7df70e-93a0-421a-a66f-818b6587e4a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_9e4af8fc-7ba2-4700-becf-ff131deb4c6a" xlink:href="mgni-20220630.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9b7df70e-93a0-421a-a66f-818b6587e4a0" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_9e4af8fc-7ba2-4700-becf-ff131deb4c6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_9df07539-8e8c-4ffb-a7c6-dbced5e12f17" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9b7df70e-93a0-421a-a66f-818b6587e4a0" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_9df07539-8e8c-4ffb-a7c6-dbced5e12f17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c0c7cf49-7069-4dd4-b604-78f02256aa69" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c0c7cf49-7069-4dd4-b604-78f02256aa69" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c0c7cf49-7069-4dd4-b604-78f02256aa69_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c0c7cf49-7069-4dd4-b604-78f02256aa69" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c0c7cf49-7069-4dd4-b604-78f02256aa69_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_461d6f98-9eec-4b5e-8602-040c417ac459" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_c0c7cf49-7069-4dd4-b604-78f02256aa69" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_461d6f98-9eec-4b5e-8602-040c417ac459" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_737fb24c-1aea-4bf5-ad72-3026e90e89f9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_461d6f98-9eec-4b5e-8602-040c417ac459" xlink:to="loc_us-gaap_ConvertibleDebtMember_737fb24c-1aea-4bf5-ad72-3026e90e89f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_044be217-bc52-48cf-b758-29ff22869640" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_461d6f98-9eec-4b5e-8602-040c417ac459" xlink:to="loc_us-gaap_SecuredDebtMember_044be217-bc52-48cf-b758-29ff22869640" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermAxis_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentConversionTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:to="loc_mgni_DebtInstrumentConversionTermAxis_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermDomain_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8_default" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentConversionTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_DebtInstrumentConversionTermAxis_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8" xlink:to="loc_mgni_DebtInstrumentConversionTermDomain_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermDomain_477adddf-0327-40e6-9057-a2dccc054e26" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentConversionTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_DebtInstrumentConversionTermAxis_0e6b7ad8-32e8-4d85-851e-8b108d1b43e8" xlink:to="loc_mgni_DebtInstrumentConversionTermDomain_477adddf-0327-40e6-9057-a2dccc054e26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIMember_0d575270-9810-4b70-ae64-7ec518160e28" xlink:href="mgni-20220630.xsd#mgni_ConversionTermIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_477adddf-0327-40e6-9057-a2dccc054e26" xlink:to="loc_mgni_ConversionTermIMember_0d575270-9810-4b70-ae64-7ec518160e28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIiMember_a660fb86-054a-4cd8-bb57-20500b8671bd" xlink:href="mgni-20220630.xsd#mgni_ConversionTermIiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_477adddf-0327-40e6-9057-a2dccc054e26" xlink:to="loc_mgni_ConversionTermIiMember_a660fb86-054a-4cd8-bb57-20500b8671bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIvMember_2cf9a837-021e-436b-a04f-b76218c49a80" xlink:href="mgni-20220630.xsd#mgni_ConversionTermIvMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_477adddf-0327-40e6-9057-a2dccc054e26" xlink:to="loc_mgni_ConversionTermIvMember_2cf9a837-021e-436b-a04f-b76218c49a80" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_2b77594f-51be-4c11-b686-f42b6b29deb6" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f5974f6b-3e61-4696-8167-9055c763c836" xlink:to="loc_srt_RangeAxis_2b77594f-51be-4c11-b686-f42b6b29deb6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2b77594f-51be-4c11-b686-f42b6b29deb6_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_2b77594f-51be-4c11-b686-f42b6b29deb6" xlink:to="loc_srt_RangeMember_2b77594f-51be-4c11-b686-f42b6b29deb6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_4f4a45c3-7bc6-453a-8e24-4fb56d5e9d29" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_2b77594f-51be-4c11-b686-f42b6b29deb6" xlink:to="loc_srt_RangeMember_4f4a45c3-7bc6-453a-8e24-4fb56d5e9d29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_07e6d9ad-c2b5-4964-8bc6-fd6640a93915" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_4f4a45c3-7bc6-453a-8e24-4fb56d5e9d29" xlink:to="loc_srt_MaximumMember_07e6d9ad-c2b5-4964-8bc6-fd6640a93915" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_cd590c3e-cad2-4daf-8df1-cb9c64bdf21d" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_4f4a45c3-7bc6-453a-8e24-4fb56d5e9d29" xlink:to="loc_srt_MinimumMember_cd590c3e-cad2-4daf-8df1-cb9c64bdf21d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails" xlink:type="extended" id="iec053d009ef5489ba73af47cfe837ee3_DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_d82dd4bd-291f-454e-b93e-d0dfde5853b8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:to="loc_us-gaap_InterestExpenseDebt_d82dd4bd-291f-454e-b93e-d0dfde5853b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_333bf43a-87cc-43a5-b8e5-8825fe50c239" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_333bf43a-87cc-43a5-b8e5-8825fe50c239" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_af7d94df-5b3f-464b-adee-4bc885820aff" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:to="loc_us-gaap_InterestExpense_af7d94df-5b3f-464b-adee-4bc885820aff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_e68584b1-3288-48be-b6dc-86b708f395ab" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_e68584b1-3288-48be-b6dc-86b708f395ab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_5abd4151-054d-4dbc-a2b3-ef3045adac80" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_924920ae-3f19-410e-8377-65ad931edb8a" xlink:to="loc_us-gaap_DebtInstrumentTable_5abd4151-054d-4dbc-a2b3-ef3045adac80" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_4c69510f-df22-486f-be90-0d4623d1ab4d" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5abd4151-054d-4dbc-a2b3-ef3045adac80" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_4c69510f-df22-486f-be90-0d4623d1ab4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_4c69510f-df22-486f-be90-0d4623d1ab4d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4c69510f-df22-486f-be90-0d4623d1ab4d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_4c69510f-df22-486f-be90-0d4623d1ab4d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ae7a978e-a67f-469c-909b-56d176a6a43f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4c69510f-df22-486f-be90-0d4623d1ab4d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ae7a978e-a67f-469c-909b-56d176a6a43f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_4f2ef273-3e77-4966-925a-1d5f3d49e44a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_ae7a978e-a67f-469c-909b-56d176a6a43f" xlink:to="loc_us-gaap_ConvertibleDebtMember_4f2ef273-3e77-4966-925a-1d5f3d49e44a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_ccfbad3d-e3b0-44ef-b062-475a0c820a85" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5abd4151-054d-4dbc-a2b3-ef3045adac80" xlink:to="loc_us-gaap_DebtInstrumentAxis_ccfbad3d-e3b0-44ef-b062-475a0c820a85" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_ccfbad3d-e3b0-44ef-b062-475a0c820a85_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_ccfbad3d-e3b0-44ef-b062-475a0c820a85" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_ccfbad3d-e3b0-44ef-b062-475a0c820a85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_67384055-5b71-48d7-800f-8aa5d8d20a70" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_ccfbad3d-e3b0-44ef-b062-475a0c820a85" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_67384055-5b71-48d7-800f-8aa5d8d20a70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_dd837d9e-6cb5-41d6-b954-d7e8eeab73a9" xlink:href="mgni-20220630.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_67384055-5b71-48d7-800f-8aa5d8d20a70" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_dd837d9e-6cb5-41d6-b954-d7e8eeab73a9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtAmortizationExpenseforDebtIssuanceCostsDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtAmortizationExpenseforDebtIssuanceCostsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtAmortizationExpenseforDebtIssuanceCostsDetails" xlink:type="extended" id="i27aa0b637d974beb8838c7201af3562f_DebtAmortizationExpenseforDebtIssuanceCostsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_35d77f81-4d77-434e-aa95-74b32277f766" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_35d77f81-4d77-434e-aa95-74b32277f766" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear_e85e9b2c-a0ac-4650-ba4b-1315e53b17a1" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear_e85e9b2c-a0ac-4650-ba4b-1315e53b17a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_ff68e4fd-e1e6-4ca1-b128-d7c76263eb34" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_ff68e4fd-e1e6-4ca1-b128-d7c76263eb34" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_6d8b2d15-3094-47d6-a269-499ee16351a9" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_6d8b2d15-3094-47d6-a269-499ee16351a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_3ddbd3bf-49be-48e1-abfe-389d47953180" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_3ddbd3bf-49be-48e1-abfe-389d47953180" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_7495b260-05cf-4367-b260-c541714203d4" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_7495b260-05cf-4367-b260-c541714203d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_db7dd616-2173-4785-9478-9f788ab3ea1c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_05e92cb3-f352-46c9-a970-ca5da7df9f37" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_db7dd616-2173-4785-9478-9f788ab3ea1c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_206bd028-4b0b-41b0-8bdf-c473a742d4c4" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_35d77f81-4d77-434e-aa95-74b32277f766" xlink:to="loc_us-gaap_DebtInstrumentTable_206bd028-4b0b-41b0-8bdf-c473a742d4c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_b759a03a-4cb3-4c8a-9416-3337a55482af" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_206bd028-4b0b-41b0-8bdf-c473a742d4c4" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_b759a03a-4cb3-4c8a-9416-3337a55482af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b759a03a-4cb3-4c8a-9416-3337a55482af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b759a03a-4cb3-4c8a-9416-3337a55482af" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b759a03a-4cb3-4c8a-9416-3337a55482af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_2b6cc8e5-cdc7-4222-9538-f3f12a78473a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b759a03a-4cb3-4c8a-9416-3337a55482af" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_2b6cc8e5-cdc7-4222-9538-f3f12a78473a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_7b5b9ab1-02b2-449d-a6b7-92962e217230" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_2b6cc8e5-cdc7-4222-9538-f3f12a78473a" xlink:to="loc_us-gaap_ConvertibleDebtMember_7b5b9ab1-02b2-449d-a6b7-92962e217230" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_b8bb3704-a05d-42ce-b0a2-051e4d57ba09" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_206bd028-4b0b-41b0-8bdf-c473a742d4c4" xlink:to="loc_us-gaap_DebtInstrumentAxis_b8bb3704-a05d-42ce-b0a2-051e4d57ba09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b8bb3704-a05d-42ce-b0a2-051e4d57ba09_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_b8bb3704-a05d-42ce-b0a2-051e4d57ba09" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b8bb3704-a05d-42ce-b0a2-051e4d57ba09_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c243f319-a7bd-4150-90c6-05708b815dea" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_b8bb3704-a05d-42ce-b0a2-051e4d57ba09" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c243f319-a7bd-4150-90c6-05708b815dea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_a8ec0a3d-452f-40a7-a7c0-156a1fa14592" xlink:href="mgni-20220630.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c243f319-a7bd-4150-90c6-05708b815dea" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_a8ec0a3d-452f-40a7-a7c0-156a1fa14592" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtCreditAgreementNarrativeDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtCreditAgreementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtCreditAgreementNarrativeDetails" xlink:type="extended" id="i297521b32e924dbd8b995060bf1cd7fd_DebtCreditAgreementNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_70dc473c-c0ff-4bdb-b5e9-b29cc75111c8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_70dc473c-c0ff-4bdb-b5e9-b29cc75111c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtTerm_bd8d2967-d509-44c5-94bf-1066a93bdcc0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_LongTermDebtTerm_bd8d2967-d509-44c5-94bf-1066a93bdcc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_b5eeb9b4-abf6-48fe-8cb1-ee2237160f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_b5eeb9b4-abf6-48fe-8cb1-ee2237160f0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDebtNetOfIssuanceCosts_6b5e7c10-8940-4c6b-bb61-f730b5d1b327" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromDebtNetOfIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_ProceedsFromDebtNetOfIssuanceCosts_6b5e7c10-8940-4c6b-bb61-f730b5d1b327" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_2022dadf-ca13-4152-9b93-36449b37d226" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_2022dadf-ca13-4152-9b93-36449b37d226" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage_88017c60-f31e-4917-b158-8482510ce8ad" xlink:href="mgni-20220630.xsd#mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage_88017c60-f31e-4917-b158-8482510ce8ad" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum_985ad1b9-0013-46ed-9c89-89a7a8af6ef1" xlink:href="mgni-20220630.xsd#mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum_985ad1b9-0013-46ed-9c89-89a7a8af6ef1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal_2bfff053-843c-4a1f-a704-f0ce9f2c8252" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal_2bfff053-843c-4a1f-a704-f0ce9f2c8252" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentAnnualPrincipalPaymentPercentage_9288ee8a-f829-4f47-93f7-cebb69c46689" xlink:href="mgni-20220630.xsd#mgni_DebtInstrumentAnnualPrincipalPaymentPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_mgni_DebtInstrumentAnnualPrincipalPaymentPercentage_9288ee8a-f829-4f47-93f7-cebb69c46689" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LineOfCreditFacilityIncreaseInMaximumBorrowingCapacity_e90299f3-e8cc-4606-b461-0c503ba046b5" xlink:href="mgni-20220630.xsd#mgni_LineOfCreditFacilityIncreaseInMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_mgni_LineOfCreditFacilityIncreaseInMaximumBorrowingCapacity_e90299f3-e8cc-4606-b461-0c503ba046b5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_0c0a5072-65c4-43f1-a4e3-9aef32a3e990" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_0c0a5072-65c4-43f1-a4e3-9aef32a3e990" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_fc361536-9aa6-4165-ae39-90f96f59db0c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_fc361536-9aa6-4165-ae39-90f96f59db0c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_f65b7eb2-c28e-4ab7-bd68-3d2eb2b537c0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_f65b7eb2-c28e-4ab7-bd68-3d2eb2b537c0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_caa0ff51-94ca-4d9b-9b7d-04a8a139206a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_caa0ff51-94ca-4d9b-9b7d-04a8a139206a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_f43e53b6-665b-4651-8b7e-655877523366" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_us-gaap_DebtInstrumentAxis_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_af5dea67-8750-4d1a-acf0-189afa9c487a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_be9c13d8-1b9f-45fe-93d1-b20a47ecca3a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_af5dea67-8750-4d1a-acf0-189afa9c487a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_c797bc87-07a7-4faa-b02d-22f04db6c282" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_af5dea67-8750-4d1a-acf0-189afa9c487a" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_c797bc87-07a7-4faa-b02d-22f04db6c282" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredRevolvingCreditFacilityMember_94cb433f-c268-4457-a9e6-352d1327b53b" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredRevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_af5dea67-8750-4d1a-acf0-189afa9c487a" xlink:to="loc_mgni_SeniorSecuredRevolvingCreditFacilityMember_94cb433f-c268-4457-a9e6-352d1327b53b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_IncrementalRevolverMember_669ac5d2-964b-4196-89dd-088447ff3c69" xlink:href="mgni-20220630.xsd#mgni_IncrementalRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_SeniorSecuredRevolvingCreditFacilityMember_94cb433f-c268-4457-a9e6-352d1327b53b" xlink:to="loc_mgni_IncrementalRevolverMember_669ac5d2-964b-4196-89dd-088447ff3c69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_9c12c9f4-d45b-4aad-bd23-e4a6a1413366" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_9c12c9f4-d45b-4aad-bd23-e4a6a1413366" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9c12c9f4-d45b-4aad-bd23-e4a6a1413366_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9c12c9f4-d45b-4aad-bd23-e4a6a1413366" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9c12c9f4-d45b-4aad-bd23-e4a6a1413366_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_26120327-df65-4318-9686-c72dabf08726" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9c12c9f4-d45b-4aad-bd23-e4a6a1413366" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_26120327-df65-4318-9686-c72dabf08726" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_a4331769-d4eb-4435-976e-4f5b650cddd7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_26120327-df65-4318-9686-c72dabf08726" xlink:to="loc_us-gaap_SecuredDebtMember_a4331769-d4eb-4435-976e-4f5b650cddd7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_fa84fc80-5d5f-4b7f-af3c-cb2750f7288e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_26120327-df65-4318-9686-c72dabf08726" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_fa84fc80-5d5f-4b7f-af3c-cb2750f7288e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_5e909dda-e430-492a-b61d-c9704c14c671" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_us-gaap_VariableRateAxis_5e909dda-e430-492a-b61d-c9704c14c671" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_5e909dda-e430-492a-b61d-c9704c14c671_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_5e909dda-e430-492a-b61d-c9704c14c671" xlink:to="loc_us-gaap_VariableRateDomain_5e909dda-e430-492a-b61d-c9704c14c671_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_46d568e0-0ff7-4c5c-b9ce-47bf9873e907" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_5e909dda-e430-492a-b61d-c9704c14c671" xlink:to="loc_us-gaap_VariableRateDomain_46d568e0-0ff7-4c5c-b9ce-47bf9873e907" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EurodollarMember_93245f07-77ca-416f-b845-fc89f6fc6060" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EurodollarMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_46d568e0-0ff7-4c5c-b9ce-47bf9873e907" xlink:to="loc_us-gaap_EurodollarMember_93245f07-77ca-416f-b845-fc89f6fc6060" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AlternateBaseRateMember_49945bd5-b3c6-4995-8548-8f96d3279ed1" xlink:href="mgni-20220630.xsd#mgni_AlternateBaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_46d568e0-0ff7-4c5c-b9ce-47bf9873e907" xlink:to="loc_mgni_AlternateBaseRateMember_49945bd5-b3c6-4995-8548-8f96d3279ed1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_590f92cf-56da-4419-9b5c-439a1b694ba2" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_srt_RangeAxis_590f92cf-56da-4419-9b5c-439a1b694ba2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_590f92cf-56da-4419-9b5c-439a1b694ba2_default" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_590f92cf-56da-4419-9b5c-439a1b694ba2" xlink:to="loc_srt_RangeMember_590f92cf-56da-4419-9b5c-439a1b694ba2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c6e8af22-811f-4c34-991a-bcb7bbe32968" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_590f92cf-56da-4419-9b5c-439a1b694ba2" xlink:to="loc_srt_RangeMember_c6e8af22-811f-4c34-991a-bcb7bbe32968" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_ad0f4ea4-6dc1-45ca-ab51-7d2ad42f7558" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c6e8af22-811f-4c34-991a-bcb7bbe32968" xlink:to="loc_srt_MinimumMember_ad0f4ea4-6dc1-45ca-ab51-7d2ad42f7558" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_114ca6d0-19d6-4c70-9732-9c9947304d2f" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c6e8af22-811f-4c34-991a-bcb7bbe32968" xlink:to="loc_srt_MaximumMember_114ca6d0-19d6-4c70-9732-9c9947304d2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_454c65ea-39e3-4569-89c1-151b9d783391" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_us-gaap_CreditFacilityAxis_454c65ea-39e3-4569-89c1-151b9d783391" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_454c65ea-39e3-4569-89c1-151b9d783391_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_454c65ea-39e3-4569-89c1-151b9d783391" xlink:to="loc_us-gaap_CreditFacilityDomain_454c65ea-39e3-4569-89c1-151b9d783391_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_9782b079-d354-42bc-8c89-04cc5a6d6c2e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_454c65ea-39e3-4569-89c1-151b9d783391" xlink:to="loc_us-gaap_CreditFacilityDomain_9782b079-d354-42bc-8c89-04cc5a6d6c2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_75c8c773-acb0-46f9-85e5-6d058b36041a" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_9782b079-d354-42bc-8c89-04cc5a6d6c2e" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_75c8c773-acb0-46f9-85e5-6d058b36041a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_77d06cd4-eb42-44ee-a865-fccbec176e46" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_9782b079-d354-42bc-8c89-04cc5a6d6c2e" xlink:to="loc_us-gaap_LetterOfCreditMember_77d06cd4-eb42-44ee-a865-fccbec176e46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsAxis_e6778014-577e-4617-81e9-0a1db4b74169" xlink:href="mgni-20220630.xsd#mgni_DebtIssuanceCostsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_8b18b06a-d755-4a3f-9286-55bd134c3484" xlink:to="loc_mgni_DebtIssuanceCostsAxis_e6778014-577e-4617-81e9-0a1db4b74169" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsDomain_e6778014-577e-4617-81e9-0a1db4b74169_default" xlink:href="mgni-20220630.xsd#mgni_DebtIssuanceCostsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_DebtIssuanceCostsAxis_e6778014-577e-4617-81e9-0a1db4b74169" xlink:to="loc_mgni_DebtIssuanceCostsDomain_e6778014-577e-4617-81e9-0a1db4b74169_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsDomain_3ef3f4cb-cba0-4384-a5b9-b2b5bdb5c1e1" xlink:href="mgni-20220630.xsd#mgni_DebtIssuanceCostsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_DebtIssuanceCostsAxis_e6778014-577e-4617-81e9-0a1db4b74169" xlink:to="loc_mgni_DebtIssuanceCostsDomain_3ef3f4cb-cba0-4384-a5b9-b2b5bdb5c1e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtDiscountMember_fe292093-c6eb-4be4-ad4b-09d93335726d" xlink:href="mgni-20220630.xsd#mgni_DebtDiscountMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtIssuanceCostsDomain_3ef3f4cb-cba0-4384-a5b9-b2b5bdb5c1e1" xlink:to="loc_mgni_DebtDiscountMember_fe292093-c6eb-4be4-ad4b-09d93335726d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DeferredFinancingCostsMember_60098d44-aacc-450e-a4af-a84d1dae0862" xlink:href="mgni-20220630.xsd#mgni_DeferredFinancingCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtIssuanceCostsDomain_3ef3f4cb-cba0-4384-a5b9-b2b5bdb5c1e1" xlink:to="loc_mgni_DeferredFinancingCostsMember_60098d44-aacc-450e-a4af-a84d1dae0862" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtSummaryofTermLoanBFacilityDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtSummaryofTermLoanBFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtSummaryofTermLoanBFacilityDetails" xlink:type="extended" id="i1ce843e9e12e4fa1bca6aaf2d235b848_DebtSummaryofTermLoanBFacilityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_0f159de9-2805-4826-b073-8ccca6ebe309" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_0f159de9-2805-4826-b073-8ccca6ebe309" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5271daa6-7e24-4a46-b1d2-329a277c6619" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_5271daa6-7e24-4a46-b1d2-329a277c6619" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_d5281165-4583-47ca-92a3-a5f51303d9a0" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_d5281165-4583-47ca-92a3-a5f51303d9a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_2afd2a46-52d8-4077-af8c-ae140cb8deb3" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_2afd2a46-52d8-4077-af8c-ae140cb8deb3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_8243370c-1091-4347-97ab-033824b7c180" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_f9b16099-92c1-4f17-9ca1-01dd09a7a7f8" xlink:to="loc_us-gaap_DebtInstrumentTable_8243370c-1091-4347-97ab-033824b7c180" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8243370c-1091-4347-97ab-033824b7c180" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_a5951cb0-549d-44ce-b690-81c1d00671c5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e5a4b5ff-1ad0-47b7-9fa7-0ff1aa12e6d1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_a5951cb0-549d-44ce-b690-81c1d00671c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_4d9494d0-034c-440c-97ed-a3c3acb4306f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_a5951cb0-549d-44ce-b690-81c1d00671c5" xlink:to="loc_us-gaap_SecuredDebtMember_4d9494d0-034c-440c-97ed-a3c3acb4306f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_7e86efd5-92f5-4116-abf2-c1037555c57b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8243370c-1091-4347-97ab-033824b7c180" xlink:to="loc_us-gaap_DebtInstrumentAxis_7e86efd5-92f5-4116-abf2-c1037555c57b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7e86efd5-92f5-4116-abf2-c1037555c57b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_7e86efd5-92f5-4116-abf2-c1037555c57b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7e86efd5-92f5-4116-abf2-c1037555c57b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_cc1eca2b-fe94-4ae4-9271-cfc6c732a8a8" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_7e86efd5-92f5-4116-abf2-c1037555c57b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_cc1eca2b-fe94-4ae4-9271-cfc6c732a8a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_cb137df6-89c9-455c-bfa9-f8f31737c5b6" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_cc1eca2b-fe94-4ae4-9271-cfc6c732a8a8" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_cb137df6-89c9-455c-bfa9-f8f31737c5b6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails" xlink:type="extended" id="i070dc95137ee4df8aaa6591ecf01aab3_DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_62dca199-fa77-4f35-a8cb-d894b74471e5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_InterestExpenseDebt_62dca199-fa77-4f35-a8cb-d894b74471e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_b8bc5f42-c9e6-424b-bcfb-150905d9b6a9" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AmortizationOfDebtDiscountPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_b8bc5f42-c9e6-424b-bcfb-150905d9b6a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_1becd5ab-1873-4b5f-9951-25e004ad79cb" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_1becd5ab-1873-4b5f-9951-25e004ad79cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_fa520f18-1503-4873-bcd4-f660f643645f" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_InterestExpense_fa520f18-1503-4873-bcd4-f660f643645f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_94523b9e-9720-42a2-8731-4fa2945e7a98" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_94523b9e-9720-42a2-8731-4fa2945e7a98" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_a49dd1f0-6d83-45eb-8545-3d0ccdce5df5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_9f4f98bc-bedf-4d15-9bdc-dca7417008e7" xlink:to="loc_us-gaap_DebtInstrumentTable_a49dd1f0-6d83-45eb-8545-3d0ccdce5df5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_a49dd1f0-6d83-45eb-8545-3d0ccdce5df5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_feaa46c1-4b56-4448-bbe9-fce2f36a6836" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_1f3bb59c-c15c-44bb-be2d-6b1e17a5077b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_feaa46c1-4b56-4448-bbe9-fce2f36a6836" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_44bc8211-056c-4c25-9703-4476b42ff951" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_feaa46c1-4b56-4448-bbe9-fce2f36a6836" xlink:to="loc_us-gaap_SecuredDebtMember_44bc8211-056c-4c25-9703-4476b42ff951" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_a214a3e8-2d6b-4e38-9106-833b075c6060" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_a49dd1f0-6d83-45eb-8545-3d0ccdce5df5" xlink:to="loc_us-gaap_DebtInstrumentAxis_a214a3e8-2d6b-4e38-9106-833b075c6060" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a214a3e8-2d6b-4e38-9106-833b075c6060_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_a214a3e8-2d6b-4e38-9106-833b075c6060" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a214a3e8-2d6b-4e38-9106-833b075c6060_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f61f5908-8a43-43eb-b19c-e4cd86c51e49" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_a214a3e8-2d6b-4e38-9106-833b075c6060" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f61f5908-8a43-43eb-b19c-e4cd86c51e49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_5834f19b-a2ae-46e1-bde4-95e50672e31b" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_f61f5908-8a43-43eb-b19c-e4cd86c51e49" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_5834f19b-a2ae-46e1-bde4-95e50672e31b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails" xlink:type="simple" xlink:href="mgni-20220630.xsd#DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails" xlink:type="extended" id="iec001782fffd4a97a68e2e496c250b0b_DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_387da209-3c0b-4a57-a7da-e5bff3283672" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_387da209-3c0b-4a57-a7da-e5bff3283672" xlink:to="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumRemainderOfFiscalYear_8149631c-bf16-47dc-86ec-66332ef0e831" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumRemainderOfFiscalYear_8149631c-bf16-47dc-86ec-66332ef0e831" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearOne_b2db966c-580c-4361-8ae4-f0e8b150e1c0" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearOne_b2db966c-580c-4361-8ae4-f0e8b150e1c0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearTwo_e61ea56c-55fa-4709-a70e-ae15c57502da" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearTwo_e61ea56c-55fa-4709-a70e-ae15c57502da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearThree_7085d605-14f0-4ca3-ba60-8ad03a00e8e4" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearThree_7085d605-14f0-4ca3-ba60-8ad03a00e8e4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearFour_9257a143-4647-4d9c-b508-d8687bb29be9" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearFour_9257a143-4647-4d9c-b508-d8687bb29be9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearFive_642bcbeb-4ab1-4b76-b1f5-b9b0ba3711e7" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtDiscountPremiumYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearFive_642bcbeb-4ab1-4b76-b1f5-b9b0ba3711e7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_fc2b286d-c7f2-4399-b220-3c106e6d5adf" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_86195e71-64ed-428f-87ec-2bb27532059b" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_fc2b286d-c7f2-4399-b220-3c106e6d5adf" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_387da209-3c0b-4a57-a7da-e5bff3283672" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear_6d8de6d2-d824-4634-8887-0e0505b2ba61" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsRemainderOfFiscalYear_6d8de6d2-d824-4634-8887-0e0505b2ba61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_68510f68-9ac0-4265-942d-9196acc4e8c2" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_68510f68-9ac0-4265-942d-9196acc4e8c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_9ef8cdab-7cc3-4fef-a1dd-bdbb0c66f840" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_9ef8cdab-7cc3-4fef-a1dd-bdbb0c66f840" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_b363c875-e831-46f0-8819-2ee31a045275" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_b363c875-e831-46f0-8819-2ee31a045275" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_2f875bee-61e5-4d4c-baa5-8f9e293f6ef7" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_2f875bee-61e5-4d4c-baa5-8f9e293f6ef7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearFive_e7b1acf0-474d-44ba-82e9-600a9c97ef9f" xlink:href="mgni-20220630.xsd#mgni_AmortizationOfDebtIssuanceCostsYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearFive_e7b1acf0-474d-44ba-82e9-600a9c97ef9f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_c48bf11a-1bb0-4778-a928-ee7fe92f73d2" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_22495420-fd0c-4945-994b-3010c818db1c" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_c48bf11a-1bb0-4778-a928-ee7fe92f73d2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_f0b97cf4-091b-40b2-b240-30610c4317f7" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_387da209-3c0b-4a57-a7da-e5bff3283672" xlink:to="loc_us-gaap_DebtInstrumentTable_f0b97cf4-091b-40b2-b240-30610c4317f7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_bef58bef-2883-482d-b7bb-c76721a988c5" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f0b97cf4-091b-40b2-b240-30610c4317f7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_bef58bef-2883-482d-b7bb-c76721a988c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_bef58bef-2883-482d-b7bb-c76721a988c5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_bef58bef-2883-482d-b7bb-c76721a988c5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_bef58bef-2883-482d-b7bb-c76721a988c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_095a58ae-be48-497c-a68d-ea38f3e54669" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_bef58bef-2883-482d-b7bb-c76721a988c5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_095a58ae-be48-497c-a68d-ea38f3e54669" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_8ea7e164-2301-406a-be34-b014c828b475" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_095a58ae-be48-497c-a68d-ea38f3e54669" xlink:to="loc_us-gaap_SecuredDebtMember_8ea7e164-2301-406a-be34-b014c828b475" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f0b97cf4-091b-40b2-b240-30610c4317f7" xlink:to="loc_us-gaap_DebtInstrumentAxis_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_3fa68186-1a5f-47b8-beae-98772ba8d93e" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_a8182239-0e00-4f8e-81a2-fdd4f8d5c26c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_3fa68186-1a5f-47b8-beae-98772ba8d93e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SeniorSecuredTermLoanFacilityMember_7f83179d-8335-4149-a0d2-5daf89675a53" xlink:href="mgni-20220630.xsd#mgni_SeniorSecuredTermLoanFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_3fa68186-1a5f-47b8-beae-98772ba8d93e" xlink:to="loc_mgni_SeniorSecuredTermLoanFacilityMember_7f83179d-8335-4149-a0d2-5daf89675a53" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
