Climate change is leading to more frequent and intense natural disasters, with developing countries particularly at risk. However, most research concerning mental health and natural disasters is based in high-income country settings. It is critically important to provide a mental health response to such events, given the negative psychosocial impacts they elicit. The aim of this systematic review is to explore the barriers and facilitators to implementing mental health and psychosocial support (MHPSS) following natural disasters in developing countries. Eight databases were searched for relevant quantitative and qualitative studies from developing countries. Only studies reporting barriers and/or facilitators to delivering MHPSS in response to natural disasters in a low- or middle-income country were included and full texts were critically appraised using the McGill University Mixed Methods Appraisal Tool. Reported barriers and facilitators were extracted and analysed thematically. Thirty-seven studies were included in the review, reflecting a range of natural disaster settings and developing countries. Barriers to implementing MHPSS included cultural relevance, resources for mental health, accessibility, disaster specific factors and mental health stigma. Facilitators identified included social support, cultural relevance and task-sharing approaches. A number of practical approaches can be used to facilitate the implementation of MHPSS in developing country settings. However, more research is needed on MHPSS in the developing country natural disaster context, especially in Africa, and international policies and guidelines need to be re-evaluated using a decolonial lens.