Transnational municipal networks (TMNs) such as C40 or ICLEI have been posited to foster city-to-city learning in accelerating climate change mitigation and, thereby, facilitating the transition to net-zero greenhouse gas emissions. However, the existing literature on the role of climate networks has hardly examined the relationship between membership and climate change mitigation outcomes and impact, without which it is premature to be optimistic about TMNs role in the net-zero transition. In this article, we address this gap through a mixed methods analysis in the case of the C40 cities initiative. We combine a staggered difference-in-differences regression to shed light on the relationship between membership in the C40 initiative and carbon dioxide (CO₂) emissions during 2002–18 in over 700 OECD cities with a qualitative cross-case analysis of Bogotá, Colombia and Copenhagen, Denmark to unpack how and when the C40 initiative influences climate action at the city level. Results show that there is no statistically significant relationship between C40 membership and CO₂ emissions, indicating that cities in the C40 initiative may not have reduced CO₂ emissions more than other OECD cities, after controlling for socioeconomic characteristics, weather, country characteristics, city fixed effects, time fixed effects, and city-specific annual time trends. Furthermore, the complementary qualitative analysis showed the C40 network's direct intervention is limited to increasing or maintaining the ambition of cities; the network was found to have limited influence on city-level policy planning and implementation. There is a need to further study and address local policy implementation for realizing net-zero in relation to TMN membership.