OBJECTIVE: Mandatory enrollment into Medicaid-contracted health maintenance organizations (HMOs) is the most common form of Medicaid managed care (MMC), but the effects of this enrollment on children are unclear. We leveraged variation in MMC implementation within and across states over time to examine the effect of mandatory Medicaid HMO enrollment on children's access, utilization, and health outcomes. METHODS: Using Medical Expenditure Panel Survey data from 2000 to 2018 and multivariable regression models, we estimated the effects of living in a county with mandatory Medicaid HMO enrollment only, compared to other MMC types and fee-for-service (FFS) combined in 1 comparison group, on outcomes for children under 18 years. We also evaluated potential effect heterogeneity across age, race and ethnicity, and for children with special health care needs (CSHCNs). RESULTS: There were small and nonsignificant associations between mandatory HMO enrollment and most outcome measures. However, mandatory HMO enrollment was associated with a 1.8%-point decline in the likelihood of having a usual source of care, 95% confidence interval (CI) [-0.035, -0.001], a 4.2%-point increase in the likelihood of delayed access to care, 95% CI [0.012, 0.072], and a 2.2%-point reduction in the likelihood of having any outpatient physician visits, 95% CI [-0.043, -0.0004], compared to other MMC and FFS combined in 1 group. Mandatory HMO enrollment was associated with more difficulty seeing a specialist for CSHCNs. CONCLUSIONS: Overall, there is little evidence that mandatory Medicaid HMO enrollment has discernable and consistent effects across a broad range of outcomes. Evaluating how mandatory Medicaid HMOs affect more nuanced health care measures, especially for children with greater health care needs, remains an important future research question.