Introduction Evidence suggests group walking in natural environments is more beneficial to the general population’s mental health than walking indoors, in urban environments, and alone. Such ‘green walking groups’ have been suggested as an occupational therapy intervention that could be suitable for adults with mental health problems. However, there have been no reviews of the mental health outcomes of participating in green walking groups for this population. Method A mixed-methods literature review was conducted. A range of databases was systematically searched electronically. Papers that met pre-defined inclusion criteria were selected, critically appraised, and qualitative and quantitative data were extracted. Thematic analysis was used to identify key qualitative outcomes. Findings Six papers were included and eight mental health outcomes identified. The evidence suggests participants can experience connections with other people, connections with nature, and a sense of freedom. There is some limited evidence to support improvements to mood, self-esteem, reflection on life tasks, and symptoms of depression, with mixed evidence for experiencing a sense of achievement. Conclusion This review can be used to build the evidence base for the link between occupation and mental health, and inform the clinical decision-making of occupational therapists, who are well-placed to design and implement green walking groups.