BackgroundThe integration of mental health services into primary care is essential to improve the coverage of mental health services in low resource settings, but the evaluation of this remains challenging. We used a programme’s Theory of Change (ToC) as a conceptual framework to determine what combination(s) of conditions at facility and community level influenced the mental health service utilisation as a result of a district mental healthcare plan (MHCP) implemented in Chitwan, Nepal. In addition, we show how qualitative comparative analysis can be used to provide an integrated analysis of data from a ToC.MethodsWe conducted a longitudinal case study of 10 health facilities where the MHCP was implemented. We collected data from all facilities at baseline (October to December 2013) and quarterly following the implementation of the intervention (March 2014 to November 2016). The data were analysed using pooled qualitative comparative analysis in fsQCA V.2.5.ResultsThe following conditions were necessary for high mental health service utilisation: presence of basic and advanced psychosocial care, evidence-based identification and treatment guidelines (WHO mhGAP), referral to tertiary services and the presence of trained female community health volunteers . Two additional combinations of conditions were also identified as sufficient for a high mental health service utilisation: high medication supply, trained facility staff and either the use of a community informant detection tool or having a larger proportion of the community attend community awareness activities.ConclusionsBoth supply-side interventions (formalised approaches to health worker detection and treatment, training of health workers, supervision) and demand-side interventions (community awareness and case finding) are important to integrate mental health in primary care. ToC can be used to provide an integrated analysis of data from a ToC, therefore helping to shed light on the black box of complex multilevel interventions.