Objective: Individuals with dual diagnoses benefit from participation in mutual-help groups, though it is unclear how much such participation contributes to outcomes when accounting for utilization of treatment. Methods: We used mixed-model regressions to examine associations between participation in mutual-help groups reported at 6-month, 1-year, and 2-year follow-ups with substance use and psychiatric outcomes among outpatients with dual diagnoses (N = 304), while controlling for amounts of substance use disorder and mental health outpatient treatment. Results: Follow-up rates were 81%, 82%, and 84% at 6 months, 1 year, and 2 years, respectively. Mean involvement in mutual-help groups (scale of 0 to 14) ranged between 4.6 (SD = 4.5) and 6.1 (SD = 4.5). When controlling for baseline status and treatment amounts, more mutual-help group meeting attendance, F = 13.98, p < .001, and involvement, F = 19.32, p < .001, were associated with fewer days of alcohol use. Likewise, after controlling for baseline status and treatment amounts, more mutual-help group meeting attendance, F = 4.57, p = .03, and involvement, F = 5.94, p = .02, were associated with less drug use. Mutual-help group participation was not associated with number of psychiatric symptoms. Mental health treatment was associated with fewer days of alcohol use, F = 4.58, p = .03. Conclusions: Facilitating mutual-help group involvement among individuals with dual diagnoses, as well as attendance at more meetings, is of potential benefit to reducing alcohol and drug use.