The United Nations Framework Convention on Climate Change (UNFCCC) COP 21, including the Paris Agreement, has underlined the role of non-state-actors in limiting temperature increase to 1.5 °C compared to pre-industrial levels. Worldwide, there are an increasing number of transnational networks on climate actions driven by non-state actors. The first attempt to understand the global extent of climate actions taken by non-state actors (such as regional and municipal governments and the corporate sector) was made by a UNFCCC platform launched in 2014 called the Non-State Actor Zone for Climate Actions (NAZCA). However, different reporting frameworks of core data partners contributing to the NAZCA platform represent challenges for tracking and comparing the outcomes of transnational initiatives and their global impact. In this contribution, we focus on the two initiatives most represented in the NAZCA platform: Covenant of Mayors and Compact of Mayors, which were merged into the Global Covenant of Mayors (GCoM) in 2016. We provide a comparative analysis of the reporting frameworks, constituting the starting point of the GCoM merging process. Consequently, we review recent studies investigating their performance and identity in terms of drivers, barriers and mitigation ambition with reference to the particular case of the cities’ contribution to the 1.5 °C global warming target. In sum, there is a wealthy literature in investigating the role of the transnational initiatives in supporting cities and promoting the standards for emission accounting, while we are currently lacking a systematic knowledge on their global contribution. The new era of transnational network consolidation entails and reinforces the need for a global, comprehensive and transparent reporting framework for cities and local governments enabling to effectively contribute to the Paris Agreement.