The presence of a co-occurring substance use disorder increases the risk of incarceration for those with a serious mental illness. To examine the interface between jail and mental health systems, a sample of 1,440 individuals diagnosed with both disorders was followed for 48 months using administrative data. The majority (63%) experienced incarceration with a median length of stay of 14 days. Predictors of incarceration were a more severe substance use diagnosis and younger age. These findings inform community-wide discussions aimed at enhancing coordination and collaboration between systems with the specific goal of decreasing the high prevalence of jail interface for this population. (Contains 1 figure and 3 tables.)