PURPOSE: We aimed to explore participants' experiences of mental health during an acceptance and commitment therapy (ACT)-based guided self-help intervention to support weight management in adults with overweight or obesity during the COVID-19 pandemic (SWiM-C: Supporting Weight Management during COVID-19). METHODS: We conducted semi-structured telephone interviews with twenty participants and used reflexive thematic analysis to identify patterns of meaning across the dataset relevant to mental health. RESULTS: Four themes were conceptualized: i) Mental health changes associated with SWiM-C, ii) External factors negatively impacted mental health and intervention engagement, iii) Use and impact of coping responses, and iv) Intervention preferences based on psychological needs. CONCLUSIONS: Findings suggest that participants were exposed to multiple factors, both related to and external to the intervention, that negatively impact their mental health, yet ACT-based aspects of the SWiM-C intervention appeared to support participants to adaptively manage the decline in their mental health. The findings can be used to inform the development of future weight management interventions, such as through intervention personalization and the inclusion of more strategies that target emotional regulation.Trial registration: ISRCTN 12107048, https://www.isrctn.com/ISRCTN12107048.