Background: Little is known about what predicts disengagement from rehabilitation treatment for people affected by severe and persistent mental illness (SPMI). Aims: To identify predictors of unplanned discharge among consumers admitted to community-based residential rehabilitation units in Australia. Method: Secondary analysis of data from a prospective cohort study of consumers admitted to three Community Care Units (CCUs) between 2014 and 2017 (n = 139). CCUs provide transitional residential rehabilitation support to people affected by SPMI. Demographic, treatment-related and clinical predictors of unplanned discharge were identified using binomial regression models controlling for site-level variability. Factors associated with self- vs staff-initiated unplanned discharge were also examined. Results: 38.8% of consumers experienced unplanned discharge. Significant predictors of unplanned discharge were younger age, higher alcohol consumption and disability associated with mental illness, as well as recovery stage indicating a sense of growth and higher competence in daily task performance. 63.0% of unplanned discharges were initiated by staff, mostly for substance-related reasons (55.9%). History of trauma was more likely among consumers with self-initiated discharge than those with staff-initiated unplanned and planned discharge. Conclusions: Assertive intervention to address alcohol-use, and ensuring care is trauma-informed, may assist in reducing rates of unplanned discharge from rehabilitation care.