Regular physical activity (PA) can play a crucial role in reducing mortality and increasing the life expectancy of individuals with poor mental health and mental illness (Rosenbaum et al., 2015). Physical inactivity has been linked to higher rates of mortality and a lower life expectancy (Stubbs et al., 2017). Meeting the international recommendations of PA has important physical health (Firth et al., 2016) and mental health benefits (McDowell, MacDonncha, & Herring, 2017). However, mental health service users (MHSU) engage in significantly lower levels of PA compared to the general population. The most influential and highly cited socio-environmental barrier towards PA participation by MHSU is a lack of social support (Soundy et al., 2014). One form of social support which is gaining popularity within this population group, but is currently under researched, is peer support (Davidson, Bellamy, Guy, & Miller, 2012). Community PA programmes provide a way to facilitate social benefits through the presence of peer support, as well as enhancing the physical benefits of PA engagement. It is therefore essential to focus research on effective ways to enhance PA engagement of MHSU through community programmes which offer social support. Employing a mixed methods approach, this thesis explores peer support within an existing national community PA programme called Get Set to Go delivered by the UK mental health charity, Mind. A systematic scoping review explored the literature on peer-based community PA programmes for MHSU. A longitudinal study was conducted to explore whether social support conditions predicted change in psychological variables, before qualitatively exploring experiences of peer support (face-to-face and online) from MSHU recipients, and experiences of peer support from the perspectives of peer volunteers. Results from the longitudinal study found a significant increase in PA levels of MHSU in the face-to-face support condition, compared to a decrease in PA levels for the online social support condition. Qualitative findings highlight both positive and negative experiences discussed within key themes from MHSU perspectives (social environment for PA, shared lived experience and a supported MH journey) and the volunteers’ perspective (lived experience of mental illness and opportunities for personal development). Findings carry important research and practical implications for those designing PA programmes targeting MHSU for more effective outcomes (e.g., motivation towards PA engagement of MHSU, retention of peer volunteers and motivation towards sustained volunteering, psychological wellbeing of both MHSU and peer volunteers, and supporting the transition of MHSU into mainstream PA).