Owing to the relatively small population sizes and remoteness of the Small Island Development States (SIDS), their mental health systems face many common difficulties. These include having few mental health specialists per country, limited access to mental health services and low awareness. To overcome these limitations, the World Health Organization (WHO) Mental Health Gap Action Programme (mhGAP), which aims to decrease the treatment gap by training non-specialists, was implemented in more than 20 Pacific and English-speaking Caribbean countries. Many lessons were learnt from the experience. Mental health specialist support is crucial, and online training and supervision could be a solution. mhGAP training proved to be effective to improve knowledge and attitude, but close monitoring and supervision are needed to change clinical practice. Awareness raising and mental health service capacity building need to occur simultaneously. To realise sustainable development goals, countries need to invest more in mental health, especially in human resources; mhGAP will be one effective solution.