Patients entering an inpatient psychiatry program (N = 406) with co-occurring mental health and substance use disorders reported on their social support networks (source, type) at treatment intake, and completed symptom measures at baseline and 3-, 9-, and 15-month follow-ups (77%). Longitudinal growth models found aspects of participants’ support networks were associated with specific symptoms over time. Less family support (i.e., more conflict) was the most consistent predictor of mental health and substance use outcomes and was associated with greater psychiatric, depression, Post Traumatic Stress Disorder (PTSD), and drug use severity. More peer support (via mutual-help involvement) was associated with greater initial improvement in alcohol use severity. Findings suggest that to facilitate the benefits of social support for patients with a dual diagnosis returning to the community, specific components of support should be assessed and considered in the treatment plan, rather than viewing support as a general and undifferentiated factor affecting recovery.