Sedentary lifestyle, poor dietary behaviors and metabolic alterations associated with psychiatric medications contribute to poor health and high rates of obesity among individuals with serious mental illness (SMI). Interventions that increase engagement in physical exercise, dietary modifications, lifestyle changes and preventive health care can provide health benefits across the lifespan. These interventions have led to substantial physical improvements in some persons with SMI, while others have not improved or have experienced worsening physical health. We set out to identify characteristics of a health promotion program that persons with SMI associated with physical health improvements. Interviews were conducted with eight participants from the In SHAPE health-promotion program who lost at least 10 pounds or diminished their waist circumference by at least 10 cm. Interviews aimed to determine which aspects of the program were perceived to be most helpful in promoting physical health improvement. Among successful participants, three themes emerged, highlighting the importance of: (i) individualized interventions promoting engagement in the program; (ii) relationships with health-promotion program employees and (iii) self-confidence resulting from program participation. Health-promotion programs that target these areas may have better success in achieving health benefits for persons with SMI.