BACKGROUND: To facilitate decentralisation and scale-up of mental health services, Fiji's Ministry of Health and Medical Services committed to implementing the World Health Organization's mental health Gap Action Programme (mhGAP). mhGAP training has been prolific; however, it remains unclear, beyond this, how successfully Fiji's national mental health program has been implemented. We aim to evaluate Fiji's mental health program to inform Fiji's national mental health program and to develop an evidence-base for best practice. METHODS: The study design was guided by the National Implementation Research Network and adhered to the Consolidated Framework for Implementation Research. CFIR constructs were selected to reflect the objectives of this study and were adapted where contextually necessary. A mixed-methods design utilised a series of instruments designed to collect data from healthworkers who had undertaken mhGAP training, senior management staff, health facilities and administrative data. RESULTS: A total of 66 participants were included in this study. Positive findings include that mhGAP was considered valuable and easy to use, and that health workers who deliver mental health services had a reasonable level of knowledge and willingness to change. Identified weaknesses and opportunities for implementation and system strengthening included the need for improved planning and leadership. CONCLUSION: This evaluation has unpacked the various implementation processes associated with mhGAP and has simultaneously identified targets for change within the broader mental health system. Notably, the creation of an enabling context is crucial. If Fiji acts upon the findings of this evaluation, it has the opportunity to not only develop effective mental health services in Fiji but to be a role model for other countries in how to successfully implement mhGAP.