This paper explores the institutional mainstreaming of nature-based solutions (NBS) to advance a process-based understanding about how to strategically develop the governance capacities needed for systemic, localised and inclusive NBS. To this end, it reports how policy officers in ten European cities have started to mainstream NBS by interacting with and changing incumbent governance arrangements when experimenting with novel governance processes and mechanisms to plan, deliver and steward NBS. The analysis identifies three mainstreaming strategies to develop capacities for (a) a systems’ approach to link NBS to policies, regulations, and departments across goals and sectors, (b) inclusive collaborations for localised and inclusive interve...