The community and provider-driven social accountability intervention (CAPSAI) project was a complex intervention consisting of interacting components, actors and processes, implemented in Ghana and Tanzania from 2018 to 2020. It aimed to measure the impact of a social accountability intervention implemented in family planning services, on contraceptive uptake and use. The intervention incorporated a process evaluation to identify key causal pathways and better understand factors influencing uptake and use, in the context in which the intervention was implemented. This study used a unique methodology to conduct the process evaluation, and looked at several outcomes, including service utilisation, continuation rates and attitudes and behaviours. Qualitative methods were used to evaluate the effects of the intervention – (1) context mapping, (2) a process evaluation of the different steps of the intervention, and (3) compiling cases of change. Document review, non-participant observation and in-depth interviews were conducted. Data analysis included summaries and coding of the qualitative data. Coding and thematic content analysis were conducted collaboratively across two countries and used a single codebook that was relevant to both countries, yet enabled country specific analysis. Each country team drafted reports that drew on the three data sets. Various lessons were learnt – including the importance of teamwork, development of standard operating procedures to guide practical study components, and how to prioritise study activities over a long study timeline. The study demonstrated that a process evaluation of a complex intervention across multiple countries is feasible, and was also able to yield differentiate results based on specificity in each country’s context. Based on this study, similar process evaluation activities can be replicated in future work.