The Affordable Care Act (ACA) required that insurers allow people ages 19-25 to remain as dependents on their parents' health insurance beginning in 2010. Using data from the 2008-12 National Survey of Drug Use and Health, we examined the impact of the ACA dependent coverage provision on people ages 18-25 with possible mental health or substance use disorders. We found that after implementation of the ACA provision, among people ages 18-25 with possible mental health disorders, mental health treatment increased by 5.3 percentage points relative to a comparison group of similar people ages 26-35. Smaller, but consistent, effects were found among all young adults, not only those with possible illnesses. For people using mental health treatment, uninsured visits declined by 12.4 percentage points, and visits paid by private insurance increased by 12.9 percentage points. We observed no changes in mental health treatment setting. Outcomes related to substance abuse treatment did not change during the study period. The dependent coverage provision can contribute to a broader strategy for improving behavioral health treatment for young adults.