Aims: To measure the immediate impact of participating in arts-on-prescription workshops on multiple dimensions of mood and to evaluate whether improvement in mood is a mechanism for change, predicting improvements in global wellbeing before and after participation in arts-on-prescription programmes. Methods: The evaluation drew upon the experience sampling method, asking participants to complete a six-item mood questionnaire at the beginning and end of each workshop in a 12-week-long arts-on-prescription programme. Participants also completed a measure of global wellbeing at the beginning and end of the programme. Results: Multilevel modelling was used to test hypotheses since the data were hierarchical (with 1491 mood reports nested within 66 participants). There was a significant improvement in global wellbeing across participation in the arts-on-prescription programme. After each art workshop there was a significant increase on all dimensions of mood: hedonic tone (contentment); tense arousal (calmness); and energetic arousal (alertness). There was also a significant improvement in these dimensions of mood, over time, upon arrival at the art workshops each week. Furthermore, reduction in tense arousal after art workshops significantly predicted changes in global wellbeing. Conclusion: The findings suggest that a reduction in tense arousal (feeling less nervous, anxious and stressed) is a crucial component of arts-on-prescription services and make a direct link between experiences during art workshops and changes in global wellbeing for the first time. This strengthens the evidence base for arts-on-prescription and suggests that tracking experience across interventions is a useful evaluation tool, with much potential.