In 2009, 98.0% of people with mental illness in Sierra Leone were not receiving treatment, partly due to the absence of public psychiatric facilities outside the capital. In response to this situation, the Ministry of Health and Sanitation rolled out nurse-led mental health units (MHUs) to every district. This study evaluates the barriers and facilitators to mental health service delivery in decentralised MHUs in Sierra Leone using key informant interviews and focus group discussions with 13 purposefully sampled clinical staff and senior management personnel. The interviews were audio-recorded, translated from Krio if necessary, transcribed, and analysed using manifest content analysis. The findings suggest that factors affecting nurse-led mental health service delivery include small workforce and high workload, culture and beliefs, risks, lack of safety measures and required resources, outdated policies, poor salaries, lack of funds for medication, distance, power, influence, and stigma. Factors that could facilitate nurse-led mental health services include: increasing motivation, increasing the workforce, knowledge sharing, mentorship, availability of medication, passion and modern psychiatry. The findings contribute towards understanding the challenges and opportunities faced by the recently established nurse-led decentralised mental health services across Sierra Leone, in order to address the large mental health treatment gap. We hope the findings will inform further policy and planning to improve the quality of decentralised mental healthcare.