This study aimed to identify substance use initiation among justice-involved adolescents transitioning into adulthood. Lifetime use of 11 substances was extracted from the U.S. Pathways to Desistance Study (N = 1,354) and modeled using latent class/transition analyses. Users were categorized into five classes: no/occasional use of alcohol and cannabis; alcohol, cigarette, and cannabis; stimulants; sedatives and hallucinogenic drugs; and all substance use. Justice-involved youths initiate substance use very early on, with substance users having already initiated substances between ages 16 and 23 on average. Those who used few substances at age 16 on average were likely to initiate illicit substances before age 23. Our findings support the importance of ensuring timely access to substance use prevention for this vulnerable population.