Background: Long-term adherence to physical activity (PA) following a cardiac event is uncommon. Interventions aiming to address this issue must take into account the influences and motivations of successful long-term adherers and strive to utilise available resources to ensure cost-effectiveness and sustainability. The purpose of this study was to examine factors influencing sustained adherence to an established Phase IV community-based cardiac rehabilitation programme: HeartSmart. This information was used to design, implement and evaluate an intervention to increase adherence among programme newcomers. Methods: Twenty-four long-term adherers of HeartSmart (N=15 men, 9 women; aged 67.7 years ± 16.7) took part in focus group discussions to determine factors assisting adherence. Results influenced the development of a Peer Mentor (PM) training programme. Eight long-term adherers (100% male, 64-77yrs) completed PM training (8 hours) covering social support, self-efficacy, benefits and barriers of PA adherence and goal setting. The PM role was to provide support, during 2 weekly exercise sessions, to programme newcomers (Mentees). Mentees (N=13, 82% male, 50-77yrs), who had recently experienced a cardiac event were recruited and paired with a PM. Outcome measures included focus groups with PMs and 1-1 interviews with Mentees. Attendance rates, psychosocial correlates (Mentees only) and PA levels were also measured. Results: Social support, elements of the structured class, health benefits and self-efficacy were the strongest influences of long-term adherers of HeartSmart. PMs rated training highly and reported a positive experience in the role. Challenges identified included assistance with exercise mastery and gauging support required by the Mentees. Mentees: Eight Mentees (7 male) were still attending HeartSmart at 6-weeks (mean 67% adherence) and reported positive experiences of the PM intervention. Reasons for dropout included injury (N=2), illness (1), pace too challenging (N=1) and feeling too young for the class (N=1). Conclusion: The intervention demonstrated positive results for both PMs and Mentees. It helped newcomers ‘fit in’ to an ‘old programme’. However, peer mentoring alone was insufficient to address adherence issues for all participants; future research needs to examine this problem further.