To inform early intervention, this study describes correlates of substance use among young people with serious mental illness (SMI) enrolled in integrated care in community mental health settings. 227 adults ages 18-35 were assessed for clinical characteristics and substance use. Logistic regressions were used to describe relationships between substance use and participant characteristics. Over a third (38.9%) reported daily cannabis, 15.9% past month other illicit drug, 13.5% frequent/heavy alcohol and 47.4% any of these; 50.2% reported daily tobacco smoking and 23.3% current vaping. Daily cannabis and tobacco were the most common combination. Alcohol, drug, and cannabis with tobacco were associated with higher mental health symptoms but not with emergency room or hospital utilization. Cannabis and other substance use was common and associated with higher symptoms but not with greater hospital utilization, suggesting that early intervention could prevent long-term negative consequences.