The purposes of this quasi-experimental pilot study were to determine adherence to a 12-week group-based moderate-intensity walking program for sedentary adult outpatients with serious and persistent mental illness and to examine change from baseline to after the walking program in health status (mental and physical health, mood, and psychosocial functioning) and exercise motivation (exercise outcomes expectancies, exercise decisional balance). The 15 volunteers in this study were aged 21 to 65 years and enrolled in psychosocial rehabilitation; they participated in a 12-week walking program meeting three times per week for 1 hr, supplemented with four health information workshops delivered at the beginning of the study. Participants received individual exercise prescriptions determined by preprogram fitness testing and used heart rate monitors during walking sessions. Thirteen participants (87%) completed the study and attended 76% of the walking sessions. Overall, they walked at lower intensity than prescribed, with pulses within target heart rate ranges 35% of the time during Weeks 1 through 4, 26% of the time during Weeks 5 through 8, and 22% of the time during Weeks 9 through 12. However, mood improved (Profile of Mood States, t = -2.51, two-tailed, df = 12, p = .02), as did psychosocial functioning (Multnomah Community Ability Scale, two-tailed, df = 12, t = 2.49, p = .02). The findings indicate a walking group may be feasible for rehabilitation programs. In addition to the known cardiovascular risk-reduction benefits of regular walking, walking may improve mood and psychosocial functioning in adults with serious and persistent mental illness.