Background: Mental illness is a major public health concern in the United States. Information on the prevalence of mental illness is needed to help inform effective treatment and prevention programs. Method: Combined 2012–2014 National Survey on Drug Use and Health national, regional, state-level, and substate-level estimates of serious mental illness (SMI) among adults aged 18 or older were analyzed. Results: Nationally, 9.3 million adults aged 18 or older experienced SMI in the past year, corresponding to 4.13 percent of the adult population. Across the census regions, estimates of past year SMI were 4.32 percent in the Midwest, 4.16 percent in the South, 4.09 percent in the West, and 3.86 percent in the Northeast. In the Midwest, estimates of past year SMI ranged from 4.87 percent in Missouri to 3.42 percent in Illinois. In the South, estimates of past year SMI ranged from 5.53 percent in West Virginia to 3.31 percent in Maryland. In the West, estimates of past year SMI ranged from 5.17 percent in Utah to 3.43 percent in Hawaii. In the Northeast, estimates of past year SMI ranged from 5.28 percent in Vermont to 3.43 percent in Connecticut. Only 1 state, North Carolina, experienced a statistically significant increase from 2010–2012 to 2012–2014 in the rate of past year SMI for adults, while the remaining 49 states and the District of Columbia experienced no change in the percentage of adults with past year SMI. Conclusion: Highlighting the percentage of people with SMI at state and substate levels can help policymakers inform their assessments of mental health needs in their communities