Introduction: The health status and psychosocial needs of the Medicaid expansion population have been estimated but not measured. This population includes childless adults predicted to have high rates of mental illness, especially among the homeless. Given limitations in access to mental health services, it is unclear how prepared the U.S. health care system is to care for the needs of the expansion population. Method: Using enrollment and claims data from the Minnesota Department of Human Services, this study presents prevalence rates of mental illness diagnoses and measures of unstable housing in Minnesota’s childless-adult early Medicaid expansion population. Rates are compared with prior predictions of serious psychological distress and mental illness constructed from the National Survey on Drug Use and Health (NSDUH) using χ2 and t tests. Results: Diagnoses of mental illness in Minnesota’s childless-adult early Medicaid expansion population were more than 15% higher than prevalence measures of mental illness/distress for the current Medicaid population. Diagnosis rates fell within confidence intervals of estimates of mental illness for Minnesota’s Medicaid expansion population. Almost 1 in 3 enrollees had a marker of unstable housing; of this group, half had mental illness and/or distress. Discussion: Findings support predictions of the high burden of mental illness and unstable housing among the Medicaid expansion population. Minnesota offers lessons to other regions working to care for such populations: (a) the use of flexible financing structures to build integrated care systems and (b) passage of legislation to allow data sharing among mental health, social services, and medical care. (PsycINFO Database Record (c) 2016 APA, all rights reserved)