Addressing international sustainable development is an urgent and critical challenge. Existing research has focused on the role of individual intermediary organizations in supporting and shaping inclusive local markets to enhance development. However, it has overlooked how clusters of organizations act as intermediaries to build development capacity, and how they do so through local public institutions. This study focuses on the United Nations (UN) system and examines a specific transnational governance arrangement (Delivering as One) that envisioned the creation of local clusters of UN organizations to facilitate institutional capacity-building in developing countries. By comparing the evolution of the eight pilot UN arrangements, we distil a model of how intermediary clusters facilitate local capacity-building through a process of contextual bridging driven by interorganizational collaboration within the cluster. Our findings identify four mechanisms of contextual bridging. The first two—functional amalgamation of operations and development of a cohesive collective voice—strengthened internal collaboration among cluster members; subsequently, sharing authority with local stakeholders and the local calibration of interorganizational collaboration (IOC) allowed the entrenchment of IOC into the local context. Our work contributes to the literature on interorganizational collaboration and transnational governance in the face of global challenges.