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Background: The sustainability of integrating mental health into primary health care (PHC) in low- and middle-income countries (LMICs) is uncertain, often failing beyond pilot stages. This study examines the policy and governance factors shaping the sustainability of the Mental Health in Primary Care (MeHPriC) project in Lagos, Nigeria, a state-led MNS integration initiative. Methods: This qualitative study combined a systematic document review with 15 in-depth interviews with state policymakers and MeHPriC managers. Guided by the Integrated Sustainability Framework (ISF) and Supporting the Use of Research Evidence (SURE) framework, we explored how policy, financing, and coordination influence sustainability. Data were analysed thematically and triangulated to map implementation pathways. Findings: Lagos State demonstrated strong formal commitment via policies, task-shifting guidelines, and a mental health law. However, sustainability was constrained by implementation gaps, weak supervision, inconsistent budget execution, and high workforce turnover. Leadership enabled early momentum, but coordination was limited and informal. Community engagement diminished post-investment. Findings suggest sustainability depends on enforcement, ownership, and embedded financing and accountability mechanisms, not just policy presence. Interpretation: Sustainable mental health integration requires more than scaling clinical protocols; it demands structural alignment across policy, leadership, financing, and community domains. The MeHPriC case underscores the importance of treating sustainability as a deliberate, systemic priority from the outset. Policymakers in LMICs must institutionalise governance and financing pathways to ensure long-term resilience. Funding: This study was funded by the UK Medical Research Council (MRC) under the Global Health Systems Research programme Grant number: MR/T021845/1.
Adewuya et al. (Sun,) studied this question.