OBJECTIVES: Bereavement is associated with negative affective, cognitive, behavioural and physiological responses. However, factors, such as coping, self-efficacy and self-esteem, can buffer negative effects of grief, and can be increased through mutual support interventions, such as shared leisure activities. This study used a non-randomised controlled design to explore the effects of group choir singing on mental health among people who have been bereaved due to cancer. METHODS: A total of 58 adults bereaved in the last 5 years who had not started psychological therapy in the last 12 weeks or medication for anxiety or depression in the last month were recruited and elected to join a choir (n=29) or participate in the non-intervention control group (n=29). Joining a choir involved engaging in 90 min weekly singing and social sessions for 12 weeks with a post-intervention assessment at week 24. We used linear mixed effects models adjusted for demographics, health-related variables, musical engagement and time since bereavement to model changes over time between the two groups in symptoms of anxiety, depression, well-being, self-efficacy and self-esteem. RESULTS: Participants who sang in a choir had more stable symptoms of depression and levels of well-being, as well as gradual improvements in their sense of self-efficacy and self-esteem over the 24 weeks. In contrast, those in the control group showed gradual increases in depressive symptoms, reductions in levels of well-being and self-esteem and no improvement in their self-efficacy. These results were independent of all covariates. CONCLUSIONS: Weekly group singing could be a promising mutual support intervention for people experiencing grief. TRIAL REGISTRATION NUMBER: NCT02756780.