Background: Mobile instant messaging (MIM) tools, such as WhatsApp, have transformed global communication practice. In the field of global health, MIM is an increasingly used, but little understood, phenomenon. Objectives: It remains unclear how MIM can be used by rural community health workers (CHWs) and their facilitators, and what are the associated benefits and constraints. To address this gap, WhatsApp groups were implemented and researched in a rural setting in Malawi. Methods: The multi-site case study research triangulated interviews and focus groups of CHWs and facilitators with the thematic qualitative analysis of the actual conversations on WhatsApp. A survey with open questions and the quantitative analysis of WhatsApp conversations were used as supplementary triangulation sources. Results: The use of MIM was differentiated according to instrumental (e.g. mobilising health resources) and participatory purposes (e.g. the enactment of emphatic ties). The identified benefits were centred on the enhanced ease and quality of communication of a geographically distributed health workforce, and the heightened connectedness of a professionally isolated health workforce. Alongside minor technical and connectivity issues, the main challenge for the CHWs was to negotiate divergent expectations regarding the social versus the instrumental use of the space. Conclusions: Despite some challenges and constraints, the implementation of WhatsApp was received positively by the CHWs and it was found to be a useful tool to support distributed rural health work.