Objective: Obesity is associated with co-morbid mental illness. The Canberra Obesity Management Service (OMS) supports adults with severe obesity who have the psychosocial capacity to engage. This study will determine whether mental illness is a predictor of OMS attendance and anthropometric changes. Method: A retrospective audit was performed from July 2016 to June 2017. Baseline characteristics, attendance and anthropometrics were stratified according to the presence of mental illness. Outcomes included weight stabilisation and clinically significant weight loss. Descriptive analyses were performed. Results: Mental illness was present in 60/162 patients (37%). Attendance was similar for those with and without mental illness. Patients with mental illness had twice as many co-morbidities ( p = .001). Depressive disorders were most common ( n = 28, 47%). Anxiety, schizophrenia spectrum and other psychotic disorders, and trauma- and stressor-related disorders also featured. Weight stabilisation was achieved by 25 patients (66%) with mental illness and 25 (35%) without. Clinically significant weight loss was observed in 10 patients (26%) with and 26 (40%) without mental illness. Conclusion: The presence of mental illness did not impact OMS attendance or weight stabilisation. The higher rate of co-morbidities in those with mental illness highlights the challenges faced by this vulnerable population.