This study objectively measured physical activity levels in overweight and obese adults with severe mental illness and examined relationships among psychiatric symptoms, cognitive functioning and physical activity. A diverse sample (50% female, 50% African American) of overweight and obese adults (n =55) with mental illness were asked to wear accelerometers for 4 days. Study participants averaged 120 min/week of moderate to vigorous physical activity (MVPA); 35% had ≥150 min/week of MVPA. Only 4% accumulated ≥150 min/week of MVPA in bouts ≥10 min as per public health recommendations. Depressive symptoms, psychological distress and cognitive functioning were not associated with physical activity (p >.05). Although participants appeared to have substantial minutes of MVPA, increased physical activity bouts, or sessions, may be necessary for increased health and weight management benefits for persons with severe mental illness. Efforts are needed to increase physical activity sessions in this vulnerable population.