Research is limited regarding the role of high-risk behaviors, trauma, and posttraumatic stress disorder (PTSD) symptoms in people with severe mental illnesses (SMI). The current survey of 276 community mental health clients diagnosed with either a schizophrenia spectrum disorder or a major mood disorder examined the mediating role of lifetime high-risk behaviors with regard to lifetime traumatic events and PTSD symptom severity measured by the PTSD Symptom Scale-Interview version. Structural equation modeling revealed that lifetime high-risk behaviors had significant direct effects on PTSD symptom severity, and high-risk behaviors partially mediated the relationship between lifetime trauma and PTSD symptom severity. Lifetime trauma had the greatest total effect on PTSD symptoms, followed by primary Axis I diagnosis and lifetime high-risk behaviors. Implications for practice with people with SMI include more thorough assessments of trauma, PTSD, and high-risk behaviors and use of coping skills approaches to reduce high-risk behaviors. Limitations of the study include the cross-sectional nature of the data.