Exposure to potentially psychologically traumatic events for correctional workers is high. However, the mechanisms driving the high prevalence are relatively unexplained. Using data from a cross-sectional, online survey of correctional service workers (n = 845) in Ontario, Canada, collected in 2017–2018, we assess the prevalence of mental disorders with a specific focus on uncertainty in the workplace and between correctional roles. We find that correctional officers, institutional governance, and probation/parole officers appear most at risk of mental disorders (prevalence of any mental disorder was 56.9%, 60.3%, and 59.2%, respectively). We argue slightly lower prevalence among institutional wellness, training, and administrative staff may result in part from their more predictable work environment, where they have more control. The results reaffirm a need for evidence-based proactive mental health activities, knowledge translation, and treatment and a need to explore how authority without control (i.e., unpredictability at work) can inform employee mental health.