INTRODUCTION: The home-based Otago Exercise Programme has been shown to increase sustained physical-activity levels in older people recruited through primary care, when supported by health professionals. The ProAct65+ trial is testing this programme using volunteer peer mentors to support behaviour change. This qualitative study explored how these peer mentors experienced their role. METHODS: Ten peer mentors from the ProAct65+ trial were interviewed. Semi-structured interviews were audio-recorded, transcribed verbatim and thematically analysed. RESULTS: Peer mentors reported positive experiences including meeting new people, watching mentees progress, developing friendships and being shown gratitude for their support. Key barriers and facilitators to the mentoring process included the home and telephone as settings for support, geography and making contact with mentees. CONCLUSION: Findings from this study can help the development of peer mentor programmes in primary care for older people. Future programmes should recruit peer mentors who are local to where mentoring is needed to reduce travel difficulties.