Abstract Governments face increasing urgency to adapt to climate change. However, there is a persistent gap between needed and implemented adaptation. The implementation of adaptation often takes place at the local level, making municipalities crucial actors, particularly regarding mainstreaming adaptation into various sectors. While mainstreaming has the potential to bring many benefits, it does not necessarily result in the implementation of adaptation. Its contribution to the adaptation process over time and the resulting implementation at the local level remain poorly understood, exacerbating the adaptation implementation gap. To advance the understanding of mainstreaming for implementation, we synthesize the emerging debates on adaptation mainstreaming with the literature on environmental policy integration and offer a framework towards a nuanced conceptualization of mainstreaming. Our results from a case study of two Dutch municipalities show that managerial actions precede the acceleration of implementation in the built environment and point to a learning curve. Furthermore, mainstreaming focuses on the water sector, built environment and green infrastructure, with little attention paid to the intersection of heat stress and drought with other sectors, while differences in the implementation of adaptation in the two cases hint at the influence of the local context. We conclude that there is no blueprint to implement adaptation, and while local preferences determine the sectors where integration and implementation occur, consideration of long-term future climate change is lacking in both cities’ mainstreaming of adaptation implementation. Our framework allowed identifying the potential and pitfalls of mainstreaming adaptation towards implementation at the local level.