<p>Participation in creative arts therapies, community arts programmes delivered on prescription, and arts activities undertaken for leisure can reduce psychiatric symptomatology in people with mental health conditions. However, the literature is unclear about how and why such effects occur. In this Review, we summarize the evidence on prominent causal mechanisms underpinning the mental health impact of the arts, with a particular focus on the transdiagnostic mechanisms common across artforms. Drawing on interdisciplinary research spanning psychology, neuroscience, neurophysiology and psychophysiology, psychobiology, social science and behavioural science, we describe 50 mechanisms and organize them into broad categories to enable a focused discussion. We conclude that a combination of multiple interconnected mechanisms of action is likely to explain the positive impact of arts engagement on mental health. We propose that day-to-day behavioural patterns of arts engagement form an overall ‘arts exposome’ that activates these causal mechanisms. Priorities for the future include both developing further theoretical and practical knowledge of how specific arts interventions can be adapted to activate diagnostic-specific and transdiagnostic mechanisms and advancing understanding of how individual arts exposomes influence current and future mental health.</p>