Background: A growing body of literature demonstrates strong association between poor mental health and criminal recidivism, but research from county jails is limited. Aims: Our aim was to examine the relationship between re-arrest and severe mental illnesses-schizophrenia, bipolar disorder and major depressive disorder-together and separately and with substance use disorders, separately and as comorbid conditions, in a mid-sized county jail cohort in the southeastern United States. Methods: We examined the full cohort of 8097 individuals who were booked into the County Detention Facility between 31 January 2014 and 31 January 2015. Their incarceration data were merged with data from the local health system to investigate the presence of severe mental illness and substance use disorder diagnoses. Re-arrest data were tracked for 4 years after the index arrest. Results: Approximately 60% of the cohort was re-arrested within 4 years. People with substance use disorders, with or without severe mental illness, had higher re-arrest rates than those with severe mental illness alone or neither diagnosis. Drug-associated arrests did not explain this finding. Conclusions: Using detailed mental illness diagnosis data with a complete cohort of detained arrestees, we have shown the wide range of need among such individuals. By demonstrating that drug-associated crimes per se do not