Effective actions to mitigate climate change are urgently needed, especially in the context of cities, which are major sources of global CO2 emissions. Establishing and managing knowledge systems that integrate local knowledge can contribute to establishing more effective responses to climate change as well as transformative change towards sustainability. However, it is still unclear how new forms of urban governance should acquire, store, create, or disseminate knowledge for fostering sustainability transitions effectively. In this study, we present a multilevel knowledge system approach based on design principles informed especially by the knowledge management literature. These address (i) working environments across multiple levels, (ii) knowledge forms and types, and (iii) knowledge processes. We apply this approach to municipal climate action in the German energy transition. In particular, we focus on the operational work of municipal climate action managers of regional centers of Lower Saxony, one of the largest of the 16 federal states, and investigate their involvement in knowledge processes. Based on semi-structured interviews in 14 of the 17 regional centers, we show that structural pre-conditions for successful knowledge management and organizational learning are present. However, we also show that there is a need for improvement regarding (i) the multilevel coordination for accelerating routine operation, (ii) the persistence of local operational knowledge, and (iii) the exploitation of local innovations. Relying on these results, we offer general recommendations for municipal climate action and suggest that policies should (i) rely on local knowledge for effective decision-making, (ii) foster multilevel exchanges of explicit and tacit knowledge for implementation, and (iii) enable open-ended learning processes that leverage local innovations for creating usable transformational knowledge.