Introduction: This study examines the association between treatment in a therapeutic community for adolescents with drug and alcohol problems on hospitalisation outcomes up to 15 years later for all clients, and separately for those with and without a history of criminal conviction. Method: A quasi-experimental design was used to examine the linked administrative health and criminal justice records for all adolescents admitted to the Program for Adolescent Life Management (PALM) from January 2001 to December 2016 (n = 3059) in Sydney, Australia. ICD-10AM codes were used to designate hospitalisation outcomes as either physical injury, mental health problems, substance use disorders, or organic illness. The treatment and comparison groups were matched on factors associated with program retention, resulting in a final sample of 1266 clients. We examined the rate of hospitalisation up to 15 years posttreatment for all clients and stratified by prior conviction status using Cox regression analyses. Results: The treatment group had significantly lower rates of hospitalisation for a physical injury (HR = 0.77 [95% CI = 0.61–0.98]), mental health problem (HR = 0.62 [95% CI = 0.47–0.81]), substance use disorder (HR = 0.59 [95% CI = 0.47–0.75]), and organic illness (HR = 0.71 [95% CI = 0.55–0.92]). There was a significant interaction between treatment and prior criminal conviction status on rate of hospitalisation for physical injury, suggesting that the effect of treatment on physical injury was significantly greater for clients with a prior criminal conviction. Conclusions: Adolescents who engage in a therapeutic community treatment program may have a long-lasting reduction in the risk of subsequent hospitalisation. This also appears to apply to those with a history of criminal conviction.