Mental health service users (MHSUs) have elevated rates of cardiometabolic disturbance. Improvements occur with physical activity (PA) programs. We report the development and evaluation of three innovative peer-developed and peer-led PA programs: 1) walking; 2) fitness; and 3) yoga. Qualitative evaluation with 33 MHSUs in British Columbia, Canada, occurred. These programs yielded improvements for participants, highlighted by powerful narratives of health improvement, and improved social connections. The feasibility and acceptability of innovative peer-developed and peer-led programs were shown. Analyses revealed concepts related to engagement and change. Relating core categories, we theorize effective engagement of MHSUs requires accessibility on three levels (geographic, cost, and program flexibility) and health behavior change occurs within co-constituent relationships (to self, to peers, and to the wider community). This study highlights the benefits of peer involvement in developing and implementing PA programs and provides a theoretical framework of understanding engagement and behavior change in health programs for MHSUs.