The purpose of this pilot study was to explore the feasibility of an intervention designed to facilitate partner support for lifestyle change among overweight and obese adults with serious mental illness (SMI). Fifteen adults with SMI enrolled in a lifestyle intervention at community mental health centers participated with a self-selected partner in an additional 12-week intervention component designed to facilitate social support for health behavior change. Participants reported that the program was useful, convenient, and helped them reach their goals. Approximately two-thirds (66%) of participants were below their baseline weight at follow-up, including 27% achieving clinically significant weight loss. Participants reported significant increases in partner support for exercise and use of persuasive social support strategies. Partner support interventions that promote exercising together and positive communication may be effective for helping individuals with SMI initiate and sustain health behavior change necessary to reduce cardiovascular risk.