Recent times have seen the rediscovery and adaptation of mission-oriented innovation policies (MIPs) for driving transformative change. While such policies seek to mobilise and align stakeholders, little is known about how missions feature in policy coordination processes. We argue that to facilitate the still troublesome operationalisation of MIPs, it is essential to understand missions as ‘boundary objects’ that have some shared meanings among the participants they convene, yet are open enough to be interpreted differently by distinct actors gathering in four interconnected policy arenas—i.e. a strategic, programmatic, implementation, and performance arena. By studying the European Commission’s Horizon Europe missions, we unravel how missions as boundary objects enable and disable the coordination of heterogeneous communities. The resulting analytical perspective highlights three key mechanisms for coordinating mission meanings across communities and arenas: convergence–divergence, passage, and reflexive learning. We conclude with research avenues for studying missions as boundary objects for facilitating concerted action.