The present study assessed factors affecting patterns of pre-incarceration medical service access and use among jail detainees with serious mental illnesses and co-occurring substance use disorders. Multivariable logistic and negative binomial models controlling for socio-demographic and psychodiagnostic factors assessed the extent to which insurance status and medical need significantly affected having a regular health care place/provider and number of emergency and non-emergency care visits in the year prior to detention. The results indicated having insurance was associated with decreased emergency care use and increased access to routine medical care. In comparison with insurance status, medical need was a more important determinant of the frequency of both routine and emergency medical care visits. We believe the results broadly support Medicaid expansion under the Affordable Care Act, as well as its provisions for medical homes for offender populations.