Background Persons with mental illness are over-represented in prison populations around the world. They are more vulnerable to arrest and more likely to experience repeated encounters with the criminal justice system. Whether criminal justice involvement, in and of itself, is associated with higher mortality, particularly among offenders with mental illness, is unknown.

Methods The authors conducted a mediation analysis of mortality rates in a cohort of 79 088 offenders from British Columbia using administrative records spanning 2001–2010, where the mediating variable was the individual-level rate of criminal sentencing.

Results During 339 506 person-years of follow-up, there were 1841 deaths. The diagnosis of mental illness had no direct association with higher mortality after adjustment for confounders (HR=0.98, 95% CI 0.86 to 1.06). However, mental illness had an indirect association with mortality that was mediated through more frequent criminal justice involvement (HR=1.02, 95% CI 1.01 to 1.04).

Conclusions These findings support the hypothesis that offenders with mental illness experience higher mortality that is mediated by higher rates of criminal justice contact. The results of our study indicate that criminal justice diversion programmes are further warranted because they may contribute to the prevention of mortality among offenders with mental illness.