Community choirs can contribute to health and wellbeing, but there is less clarity about the mechanisms through which choir participation promotes positive change, or whether mechanisms are shared with other leisure activities. This research explores two possible types of mechanism: Mechanisms relating to individual characteristics pertaining to mindset and motivation, including experiences of flow, competence, autonomy, and relatedness (Study 1); and mechanisms relating to the activity experience, which include music, movement, and social opportunities (Study 2). In Study 1, middle and older aged members of choirs, exercise groups and other kinds of social groups (N = 190) completed surveys on their experience of emotional wellbeing (operationalized as positive and negative affect scores), mental wellbeing, and social cohesion (outcomes) pertaining to their group activity as well as experiences of motivation, flow, and the components of Self-Determination Theory (potential mediators). Multiple regression analyses revealed that participation in Choir or Exercise groups predicted positive emotional wellbeing, but not social cohesion. Underlying mechanisms differed, with positive affect mediated by intrinsic motivation for choir members, and by intrinsic motivation, identified regulation, and flow for Exercise group members. Mental wellbeing was correlated only with exercise group participation and was mediated by flow. Study 2 used an experience sampling methodology conducted with a sub-group from Study 1 (N = 59), which asked daily questions about wellbeing (happiness, sense of social connection, and energy levels) and participation in activities (music engagement, exercise, and social activities) experienced in their everyday lives and not directly associated with any leisure group participation. Repeated-measures t-tests revealed that participants were more likely to report higher levels of social connection on days in which they participated in music activities than on days in which they did not engage in music activities. Engaging in exercise or social activities was also associated with a greater sense of social connection, as well as higher levels of happiness and energy. In sum, the activity characteristics and individual differences of motivation and mindset towards participation correlated with greater wellbeing, reflecting an ecological model of person-activity fit, with no indication of superior benefits associated with group singing. Findings are discussed in terms of social prescribing and other settings where social opportunities are organized.