OBJECTIVE: To examine the association between Medicaid managed care (MMC) penetration and mental health (MH) service use among Medicaid-enrolled non-elderly adults, with a special focus on those with MH conditions. BACKGROUND: Medicaid covers over 9 million adults living with MH conditions, with many enrolled in MMC. Despite increases in MMC enrollment over the past decade, nationwide evidence of MMC's association with MH service use during this period is lacking. METHODS: Using 2015-2019 National Survey on Drug Use and Health data, we applied logistic and negative binomial regression models to examine the association between MMC penetration and MH service use among 35,500 non-elderly enrollees in 40 MMC states, and separately among 11,800 enrollees with MH conditions. Four dichotomous outcomes separately measured any MH service use in inpatient, outpatient, prescription medication, and any settings. Two additional count outcomes measured the number of inpatient MH stays and outpatient MH visits. RESULTS: A 2-percentage point higher level of MMC penetration was associated with a 9% reduction (adjusted incidence rate ratio = 0.91, 95% CI = 0.87, 0.94) in days of inpatient MH stays among all enrollees and a 7% reduction (adjusted incidence rate ratio= 0.93, 95% CI = 0.87, 0.99) among enrollees with MH conditions. MMC penetration was not associated with significant changes in other outcomes. CONCLUSIONS: Among non-elderly adults and those with MH conditions, increased MMC enrollment was associated with reduced inpatient MH services with no significant changes in the use in other settings. Ongoing monitoring is crucial to assess the potential impact of shortened inpatient stays on MH outcomes.