Adolescent chronic pain is a substantial public health problem, and pain symptoms often persist into adulthood. Young adults with chronic pain are at elevated risk for more frequent tobacco, alcohol and cannabis use, and cross-sectional research highlights the importance of psychosocial vulnerability factors. Limited research has examined how adolescent predictors, including mental health symptoms, pain, sleep and family functioning, impact later, young adult substance use. A prospective cohort of 229 young adults (77.3% female; Mage = 21.0, SD = 1.6) with childhood-onset chronic pain completed measurements in adolescence and a follow-up assessment in young adulthood of past 3-month substance use frequency. Adolescent sleep quality and male sex were associated with more frequent tobacco use; adolescent depression was associated with more frequent alcohol use, and adolescent pain severity was associated with less frequent, and male sex was associated with more frequent cannabis use. Adolescent predictors of young adult substance use among youth with childhood-onset chronic pain represent important factors that may inform assessment, prevention and treatment of substance use in this population. Identifying and testing psychological interventions that target these vulnerability factors may reduce overall substance use risk in young adulthood. This prospective observational study of young adults with childhood-onset chronic pain identified adolescent depression and sleep quality as vulnerability factors associated with substance use. Given the increasing risk for substance use during adolescence and young adulthood, these findings highlight the potential importance of early intervention to reduce substance use among young adults with childhood-onset chronic pain.