People with serious mental illness (SMI) are significantly overrepresented in the criminal justice system and often experience worse outcomes in the form of being rearrested for new offenses and probation/parole violations. Addressing the needs of supervisees with SMI can create unique challenges for probation and parole officers (PPOs), which in turn, may increase job related stress and impact PPOs’ mental health. However, few studies examine the stress and mental health consequences of supervising people with SMI, and how these consequences influence officers’ attitudes toward clients. Using survey and administrative data from 793 PPOs, this study assesses whether the number of supervisees with SMI on PPOs’ caseloads is associated with increased depressive symptomatology and if work stress explains this relationship. This investigation also assesses whether role conflict and role overload contribute to work stress among PPOs supervising people with SMI. Finally, this study examines the influence of PPOs’ well-being on their expectations for the success of supervisees, based on officers’ perceptions on the likelihood of supervisees with SMI committing new offenses or technical violations. This research draws from Stress Process research to better understand how supervising people with SMI relates to work stress, role conflict and overload, and mental health. Drawing from research on the Cognitive Model of Depression, this study also examines whether the relationship between supervising people with SMI and officers’ expectations of supervisee success is conditioned by officers’ mental health.