People experiencing chronic mental health conditions (CMHC) often report feeling socially marginalised. There is emerging evidence that social and mental wellbeing can be enhanced through participation in arts‐based programmes. In this paper, a social identity theoretical approach was applied to explore how participation in the arts may improve mental health in a longitudinal study. A one‐year prospective study of 34 choir members and 25 creative writing group members (M_age = 46, 51% female) with CMHC, involved three assessments of participants’ group identification and mental wellbeing, measured by the Warwick Edinburgh Mental Wellbeing Scale. The programmes were community‐based and facilitated by arts professionals. Multilevel modelling analyses demonstrated that participants’ mental wellbeing significantly improved over time. Greater identification with their arts‐based group (ABG) was significantly related to an increased rate of improvement in mental wellbeing. The trajectory of improvement in mental wellbeing did not differ between participants partaking in the choir or creative writing group. This study demonstrates that participation in ABGs can be effective in improving mental wellbeing in adults with chronic mental health problems, particularly for those who strongly identify with the group. This study supports ABG participation as an accessible component of mental health services.