This scoping review aimed to synthesise current evidence related to psycho-social groups as part of community-based mental health interventions in South Asia. We used a realist lens to pay attention to the contexts and mechanisms supporting positive outcomes. We included studies published from January 2007 to February 2022 that: were based in communities in South Asia, included a group component, reported on interventions with a clear psychosocial component, targeted adults and were implemented by lay community health workers. Two reviewers extracted data on intervention components, groups and facilitators, participant demographics and enabling contexts, mechanisms and outcomes. Expert reference panels including people with lived experience of psycho-social disability, mental health professionals and policy makers confirmed the validity and relevance of initial review findings. The review examined 15 interventions represented by 42 papers. Only four interventions were solely psycho-social and nearly all included psychoeducation and economic support. Only 8 of the 46 quantitative outcome measures used were developed in South Asia. In a context of social exclusion and limited autonomy for people with psychosocial disability, psychosocial support groups triggered five key mechanisms. Trusted relationships undergirded all mechanisms, and provided a sense of inclusion, social support and of being able to manage mental distress due to improved skills and knowledge. Over time group members felt a sense of belonging and collective strength meaning they were better able to advocate for their own well-being and address upstream social health determinants. This led to outcomes of improved mental health and social participation across the realms of intrapersonal, interpersonal and community. Psychosocial groups merit greater attention as an active ingredient in community interventions and also as an effective, relevant, acceptable and scalable platform that can promote and increase mental health in communities, through facilitation by lay community health workers.