In recent years, music-based interventions have been utilised as a tool for improving public health, reducing inequalities and promoting well-being of young people. Although some researchers have begun to draw links between music-related interventions and positive health outcomes, there is little understanding as to how such effects are produced. Realist evaluations—understanding what works, for whom and under what circumstances—are a particularly apt means by which we can open this ‘black box’. In this paper, we use a realist evaluation to assess a community-based music initiative designed and implemented to support the well-being of disadvantaged young people in Scotland. In order to gain perspectives on the range of contextual characteristics, mechanisms and outcomes, we collected quantitative and qualitative data in the form of pre- and post-questionnaires, as well as conducting interviews with beneficiaries and stakeholders. Our findings show that the intervention achieved a positive impact on the self-confidence, well-being and engagement of disadvantaged young people. This impact was achieved via an approach personally tailored to the individual needs of the young people; and an organisational environment characterised by trust, whereby young people felt safe to express themselves.