Health care is critically important as young adults transition from adolescence, building a foundation for their long-term health in adulthood. Yet, young adults ages 19 to 25 have historically had limited access to both employer-sponsored insurance and public insurance coverage, resulting in the highest rates of uninsurance among all age groups before passage of the Affordable Care Act (ACA). We examine changes in coverage and access to care among young adults between 2011 and 2018 and consider how Medicaid expansion may have contributed to observed changes in these outcomes. We investigate the impacts of the ACA Medicaid expansion, specifically, on young adult coverage and access through 2018 by comparing trends in states that did and did not expand Medicaid under the ACA. We find that ACA Medicaid expansions increased coverage among young adults and improved access to care among those with low incomes and those ages 22 to 25 without a college degree. These findings suggest expanding Medicaid in additional states could further improve coverage and access to care among young adults, particularly those who are socioeconomically disadvantaged.