Medicaid expansion narrowed racial and ethnic disparities in health coverage, but few studies have explored differential impact by exposure to structural racism. We analyzed data on historical residential redlining in US metropolitan areas from the Mapping Inequality project, along with data on uninsurance from the American Community Survey, to test whether Medicaid expansion differentially reduced uninsurance rates among nonelderly adults exposed to historical redlining. Our difference-in-differences analysis compared uninsurance rates in Medicaid expansion and nonexpansion states both before (2009-13) and after (2015-19) the state option to expand Medicaid pursuant to the Affordable Care Act took effect in 2014. We found that Medicaid expansion had the greatest impact on lowering uninsurance rates in census tracts with the highest level of redlining. Within each redline category, there were no significant differences by race and ethnicity. Our results highlight the importance of considering contextual factors, such as structural racism, when evaluating health policies. States that opt not to expand Medicaid delay progress toward health equity in historically redlined communities.