BACKGROUND: The unacceptably high rate of maternal and child mortality in Nigeria prompted the government to introduce a free maternal and child health (MCH) programme, which was stopped abruptly following a change in government. This triggered increased advocacy for sustaining MCH as a political priority in the country and led to the formation of advocacy coalitions. This study set out to explain the process involved in the formation of advocacy coalition groups and how they work to bring about sustained political prioritization for MCH in Nigeria. It will contribute to the understanding of the Nigerian MCH sector subsystem and will be beneficial to health policy advocates and public health researchers in Nigeria. METHODS: This study employed a qualitative case study approach. Data were collected using a pretested interview guide to conduct 22 in-depth interviews, while advocacy events were reviewed pro forma. The document review was analysed using the manual content analysis method, while qualitative data audiotapes were transcribed verbatim, anonymized, double-coded in MS Word using colour-coded highlights and analysed using manual thematic and framework analysis guided by the advocacy coalition framework (ACF). The ACF was used to identify the policy subsystem including the actors, their belief, coordination and resources, as well as the effects of advocacy groups on policy change. Ethics and consent approval were obtained for the study. RESULTS: The policy subsystem identified the actors and characterized the coalitions, and described their group formation processes and resources/strategies for engagement. The perceived deep core belief driving the MCH agenda is the right of an individual to health. The effects of advocacy groups on policy change were identified, along with the factors that enabled effectiveness, as well as constraints to coalition formation. External factors and triggers of coalition formation were identified to include high maternal mortality and withdrawal of the free MCH programme, while the contextual issues were the health system issues and the socioeconomic factors affecting the country. CONCLUSION: Our findings add to an increasing body of evidence that the use of ACF is beneficial in exploring how advocacy coalitions are formed and in identifying the effects of advocacy groups on policy change.