Older adults with serious mental illness (SMI) have compromised physical function that could be improved with exercise; however, retention in exercise programs is a challenge. This study was a retrospective analysis of retention for the 150 older veterans with SMI that enrolled in Gerofit, a clinical exercise program offered in the Veterans Health Administration. Chi-square and t-tests were conducted to evaluate baseline differences between those that were and were not retained at six and 12 months. Retention was 33% and better health-related quality of life and endurance were related to retention. Future work is needed to improve exercise program retention in this population.