The purpose of this study was to extend the investigation of criminal thinking of persons with mental illness beyond prison and community settings to a jail setting. Participants consisted of 122 individuals incarcerated in a county jail who were diagnosed with a severe mental illness, including schizophrenia spectrum and major mood disorders. Results indicated that people with mental illness in this sample of jail inmates presented with thinking styles that support a criminal lifestyle, and have criminal thinking styles that follow a pattern that is very similar to a sample of prison inmates with serious mental illness. These findings support the need for therapeutic programs for justice-involved persons with serious mental illness to develop a multipronged treatment approach that integrates interventions for individuals’ criminal thinking and antisocial attitudes with treatment for their mental illness and substance abuse issues.