BACKGROUND: Art interventions may provide a cost-effective approach to improving mental well-being. Most evaluations concentrate on intervention characteristics and little is known about other factors which might contribute to successful outcomes. METHODS: This pre-and-post intervention mixed-methods study explored influences on differential changes in measured well-being among participants of an Arts on Referral (AoR) scheme in the UK. Measured well-being scores of 44 volunteers and findings from six semi-structured interviews were triangulated. RESULTS: Mean well-being scores improved by 8.0 (95% CI 4.8-11.3, P < 0.0001); the number of sessions attended and baseline scores were positively associated with outcome score. Participants from Black and Minority Ethnic (BME) groups and females appeared to show greater improvement in well-being scores than White British or male participants. Qualitative interviews supported and further explained these findings and suggested differential impacts of AoR may, in part, be explained by the importance of sharing experiences, reduced social isolation and external stressors. CONCLUSION: This study supports the use of AoR interventions for improving well-being among those facing short- and long-term mental health challenges. However, given the reduced sample size and the pre-post design results should be interpreted with caution and potential differences between ethnic groups and genders should be further explored.