Cities are crucial climate change actors, developing largely voluntary action plans and emission reduction targets. There is limited evidence, however, of their impact through transnational climate initiatives and little empirical support linking mitigation strategies and emissions reductions. Here, we show that 60% of more than 1,000 EU Covenant of Mayors’ cities are on track to achieve their 2020 emission reduction targets. Assessments of cities’ mitigation outcomes and the determinants of performance show that on-track cities tend to have less-ambitious targets and higher baseline emissions and are in countries with more-ambitious national climate policies and greater realized emissions reductions than cities that are not on track. Automated textual analysis and a regression model find city emissions reduction is influenced by plan-level, city-level and country-level characteristics. Greater emissions reductions are associated with plans targeting energy efficiency. These results of city-level achievement provide empirical support for the theorized subnational actors’ contribution to global climate mitigation.