People with intellectual disabilities (ID) have experienced increasing levels of community participation since deinstitutionalization. This freedom has facilitated community inclusion, access to alcohol and drugs, and the potential for developing substance abuse (SA) disorders. People with ID, who are known to have high rates of co-occurring serious mental illness (SMI), may be especially vulnerable to the consequences of this disease and less likely to use SA treatment. Using standardized performance measures for SA treatment access (initiation, engagement), rates were examined retrospectively for Medicaid beneficiaries with ID/SA/SMI ages 12 to 99 (N = 5,099) and their counterparts with no ID/SA/SMI (N = 221,875). Guided by the sociobehavioral model of health care utilization, age-adjusted odds ratios and logistic regression models were conducted. People with ID/SA/SMI were less likely than their counterparts to access treatment. Factors associated with initiation included being nonwhite, living in a rural area, and not being dually eligible for Medicare; factors associated with engagement included all of the same and having a fee-for-service plan, a chronic SA-related disorder, or both. Social work policy and practice implications for improving the health of people with ID/SA/SMI through policy change, cross-system collaboration, and the use of integrated treatment approaches are discussed.