Background: Maternal and child health are key priorities among the Sustainable Development Goals , which include a particular focus on reducing morbidity and mortality among women of reproductive age, newborns, and children under the age of five. Two key components of maternal and child health are family planning (FP) and immunisation. Providing these services through an integrated delivery system could increase the uptake of vaccines and modern contraceptive methods (MCMs) particularly during the post-partum period. Methods: A realist evaluation was conducted in two woredas in Ethiopia to determine the key mechanisms and their triggers that drive successful implementation and service uptake of an intervention of integrated delivery of immunisations and FP. This evaluation included the: 1) development of an initial programme theory; 2) review of relevant published theories; 3) analysis of project monitoring data; 4) analysis of qualitative data from 23 semi-structured interviews to determine key mechanisms and drivers; and 5) development of a revised programme theory comprised of context-mechanism-actor-outcome and context-mechanism-intervention-actor-outcome configurations based on empirical findings. Results: The proportion of women who brought their children for immunisation and received MCMs was 63.0% (4,260/6,764) overall [74.3% (2,944/3,961) in Assosa woreda and 46.9% (1,316/2,803) in Bambasi woreda]. Key contextual factors identified were: a strong belief in values among religious leaders and community members that challenged FP; a lack of support for FP from male partners based on religious values; and the use of trained Health Extension Workers (HEWs) to deliver FP services. Within these contexts, intervention components that focused on the alignment of religious texts with FP, the promotion of FP by religious leaders, and the training of HEWs and health care workers on FP counselling and service delivery, influenced the implementation of the intervention and triggered several mechanisms of acceptability, access, and adoption of innovations. Conclusions: Linking context and intervention components to the mechanisms they triggered helped explain the intervention outcomes, and more broadly how and for who the intervention worked. This enabled the development of a programme theory that could be used to inform the integrated delivery of FP services in similar contexts.