This study examined the mediated effect of early adolescence familial context on early adulthood problematic substance use through effortful control in late adolescence. The sample consisted of a community sample of 311 adolescents and their families comprising the control group within a randomized trial intervention. Parental monitoring and parent–child relationship quality (P-C RQ) were measured annually from ages 11 to 13. Effortful control was measured by self-reports and parent and teacher reports at ages 16 to 17. Self-reports of problematic tobacco, alcohol, and marijuana use were measured at ages 18 to 19, 21 to 22, 23 to 24, and 26 to 27. Structural equation modeling was employed to test hypothesized models. Only P-C RQ was found to be significantly associated with adolescent effortful control. As expected, higher levels of adolescent effortful control were associated with lower problematic substance use through early adulthood, controlling for previous substance use levels. Mediation analyses showed that effortful control significantly mediated the relationship between P-C RQ and problematic substance use. Higher relationship quality between youth and parents in early adolescence is associated with higher effortful control, which in turn relates to a lower level of problematic substance use in early adulthood.