Abstract As community engagement (CE) is implemented for sustainable maternal and newborn health (MNH) programming, it is important to determine how these approaches work. Low- and middle-income countries (LMICs) have become a particular focus for MNH CE activities due to their high burden of maternal and neonatal deaths. MNH messaging and communication to engage communities are likely to differ by context, but how these approaches are actually developed and implemented within CE is not well understood. Understanding how communications in CE actually work is vital in the translation of learnings across programmes and to inform future projects. The purpose of this realist review is to describe how, why, to what extent and for whom communications in CE contribute to MNH programming in LMICs. After searching academic databases, grey literature and literature suggested by the expert advisory committee, documents were included if they described the CE communication processes/activities used for MNH programming in an LMIC. Relevant documents were assessed for richness (depth of insight) and rigor (trustworthiness and coherence of data/theories). Data were extracted as context–mechanism–outcome configurations (CMOCs) and synthesized into demi-regularities to contribute to theory refinement. After screening 416 records, 45 CMOCs were extracted from 11 documents. This informed five programme theories explaining that communications in CE for an MNH programme work when: communities are actively involved throughout the programme, the messaging and programme are acceptable, communication sources are trusted, the community has a reciprocal relationship with the programme and the community sees value in the programme. While these findings reflect what is often anecdotally known in CE or acknowledged in communications theory, they have implications for policy, practice and research by highlighting the importance of centring the community’s needs and priorities throughout the stages of developing and implementing communications for CE in MNH.