In the realm of healthcare digitalization, the advancement of patient-centred care models hinges crucially on the establishment of inter-organizational information infrastructures (IIs): intricate “systems of systems” designed to facilitate efficient data sharing, consolidation, and utilization. However, closed and siloed health information systems often fragment patient data across multiple repositories, resulting in inefficiencies and limitations in inter-organizational access and decision-making at the population level. Bridging these divides demands the integration of loosely coupled information systems within and across organizations. The ongoing evolution of these IIs is intricately shaped by the complex sociotechnical installed base governing their development. It can be difficult to engage potential participants in building and using an II that spans organisational boundaries particularly in contexts marked by diversity of mission, high organizational autonomy and lacking a single authoritative actor. Through a case study in England’s National Health Service, this paper examines the intricate process of mobilizing autonomous organizations to collaborate in developing a shared II for integrated care. In non-hierarchical settings, lacking a clear organizing entity such mobilization requires strategic partnerships and a mix of skills. I introduce the term “collective mobilizer” to represent the collaborative coalition of entities with the capacity to garner sufficient interest among potential constituencies and align visions and resources. By unpicking the role of collective mobilizers, this paper offers insights into the challenges of bootstrapping II development in non-hierarchical contexts and how they may be overcome.