Intersectoral action is increasingly recognized as necessary to address the social determinants of mental health. This study aims to assess South Africa's progress in intersectoral collaboration for mental health, and provide recommendations for intersectoral collaboration, to generate lessons for other low- and middle-income countries. We conducted a survey of the existing mental health system in South Africa using the World Health Organization Assessment Instrument for Mental Health Systems. We also conducted 96 semi-structured interviews and 12 focus group discussions with a range of stakeholders at national, provincial and district level. Data were analysed thematically to understand the roles and responsibilities of different sectors in realizing the right to mental health. A range of key sectors were identified as having roles in mental health promotion, illness prevention and service delivery. In discussing South Africa's progress, respondents gave several suggestions about how to formulate an intersectoral response in this context, including increasing high level political commitment, and using leadership from the health sector. We outline roles and responsibilities for various sectors and lessons that can be learnt from this context. These include the importance of developing programmes alongside legislation, employing targeted awareness-raising to engage sectors, and developing a structured approach to intersectoral action.