Managing wildfire risk across boundaries and scales is critical in fire-prone landscapes around the world, as a variety of actors undertake mitigation and response activities according to jurisdictional, conceptual and administrative boundaries, based on available human, organisational, technical and financial resources. There is a need to catalyse coordination more effectively to collectively manage wildfire risk. We interviewed 102 people across five large landscape case studies in the western United States to categorise how people and organisations were deployed in range and forestlands to collectively address wildfire risk. Across all cases, actors spanned boundaries to perform functions including: (1) convening meetings and agreements; (2) implementing projects; (3) community outreach; (4) funding support; (5) project planning; (6) scientific expertise. These functions fostered conducive boundary settings, concepts and objects to communicate and work across boundaries, navigating challenges to implementing work on the ground. This work highlights context-specific ways to advance cross-boundary wildfire risk reduction efforts and uses a boundary spanning lens to illustrate how collective action in wildfire management evolves in different settings. This research highlights prescribed fire as a gateway for future collective action on wildfire risk, including managing naturally ignited wildfires for resource benefits and improving coordination during wildfire suppression efforts.