Multiple theories suggest mechanisms by which the use of alcohol and drugs during adolescence could dampen growth in psychosocial maturity. However, scant empirical evidence exists to support this proposition. The current study tested whether alcohol and marijuana use predicted suppressed growth in psychosocial maturity among a sample of male serious juvenile offenders (n = 1,170) who were followed from ages 15 to 21 years. Alcohol and marijuana use prospectively predicted lower maturity 6 months later. Moreover, boys with the greatest increases in marijuana use showed the smallest increases in psychosocial maturity. Finally, heterogeneity in the form of age-related alcohol and marijuana trajectories was related to growth in maturity, such that only boys who decreased their alcohol and marijuana use significantly increased in psychosocial maturity. Taken together, these findings suggest that patterns of elevated alcohol and marijuana use in adolescence may suppress age-typical growth in psychosocial maturity from adolescence to young adulthood, but that effects are not necessarily permanent, because decreasing use is associated with increases in maturity.