Assertive Community Treatment (ACT) is a multidisciplinary treatment approach to support people with severe mental illnesses (SMI) in their recovery. While the majority of ACT recipients report co-morbid substance use issues, limited reviews have evaluated the impact of receiving ACT services on substance use outcomes. The purpose of this systematic review was to evaluate the impact of ACT involvement on substance use outcomes among people with SMI. A systematic literature search was conducted including articles published prior to April 2021. Twenty-nine studies were included in this review. Of them, 15 studies implemented a controlled design (six studies demonstrated high quality) and 14 studies implemented a cohort design. From a synthesis of the reviewed studies, five areas of changes associated with substance use emerged including reduced alcohol and drug use severity, lower prevalence of alcohol and drug use, increased stage of change in substance use treatment, and fewer days of hospitalization and intoxication. Thus, future studies should examine the integration of substance use treatment services as part of ACT interventions for opportunities to enhance recovery outcomes among individuals with SMI.