An increasing number of cities and local governments adhere to transnational initiatives on climate change mitigation, but quantification of their contribution to the Paris Agreement commitments is lacking. To address this, the climate mitigation trajectory to 2050 of Covenant of Mayors (CoM) signatories has been studied. As of October 2017 there are more than 7600 signatories covering 238 million inhabitants, mainly from European cities. Of these 533 have already reported on the implementation of their climate action plans and 207 signatories have declared a long term target beyond 2020. We examine the aggregated effort of the CoM signatories in terms of geographical distribution, mitigation ambition and achievements, and also discuss whether the projected emissions reductions are in line with pathways to limit global warming to 2°C or 1.5°C. Given the currently limited number of signatories presenting long-term visions, an extrapolation method has been applied to estimate of the individual trajectories to 2050. For this purpose the CoM dataset was divided in clusters on the basis of three factors: climatic features, degree of urbanization and the level ambition in pursuing CO2 emissions reductions. Results confirm the large potential of the CoM initiative to contribute to ambitious mid-century climate change mitigation targets.