We investigate the impact of the expansions of Medicaid coverage due to the Affordable Care Act (ACA) on under-served populations using claims data from the period 1999-2021. These panel data include the universe of American Indian and Alaska Native (AIAN) Medicare and Medicaid populations and a twenty percent random sample from the rest of the US which allows us to trace out enrollments and utilization of various health care services over time. We focus on AIAN individuals and the utilization of mental health and substance abuse treatment services in particular. We find an increase in the enrollment in Medicaid for the AIAN population aged 18-65 of 5-13. We also find a large increase in mental health and substance abuse prevention visits in the wake of ACA state expansions. Our results clearly indicate that Medicaid coverage increased mental health and substance-abuse related treatment in the adult (18-64) and elderly (65 and over) population of AIAN. These increases are not due to newly enrolled patients' increased utilization of health services but arise in large part from increased utilization by individuals who were already enrolled in Medicaid at the time of the expansions. Supply-side effects, such as the expansion of substance-abuse and mental health services in response to the ACA, likely account for rise in utilization. In addition to AIAN, we present evidence on African Americans' enrollment, heath care use, and outcomes for mental health and substance use disorders.