Background: This study examined the effect of a residential drug and alcohol therapeutic community program on criminal convictions among young people who had different trajectories of conviction prior to referral. Method: Data was derived from linked administrative records of all young people referred to a modified therapeutic community for serious alcohol and drug related difficulties located in New South Wales, Australia (n = 3059). Group-based trajectory modelling and propensity score matching estimated the potential treatment effect at age 17 on subsequent convictions up to 22 years of age, by three pre-referral trajectories of convictions from ages 10 to 16 years, designated “no or low convictions”, “moderate incline convictions”, and “high incline convictions”. Results: Treatment was associated with a significant decrease in convictions for only the high incline convictions trajectory after double adjustment of propensity scores and post-discharge incarceration. The reduction in the number of convictions for the high incline group was substantial (Cohen's d = 0.96), corresponding to 4.36 fewer convictions over five years post referral. Conclusions: Residential therapeutic communities may be an effective intervention to reduce future convictions among young people with drug and alcohol problems who have a high number of convictions prior to referral.