Why do some municipalities adopt ambitious climate action plans and others do not? This study examines United States cities that have signed the Global Covenant of Mayors for Climate & Energy, to identify the factors that have led some of them (37 percent, 63 cities) to adopt ambitious (Net Zero) climate action plans. It finds that two factors make the most difference: (a) whether the city has a paid city employee (or department) dedicated to environmental/energy management and (b) whether the city has a university. Other factors, such as per capita income, city revenue, state funding, size, partisan orientation, and membership in international climate networks, did not significantly influence how ambitious a city's climate action plan was. This study combines a statistical analysis of the signatory cities with a qualitative study Middletown Connecticut to explain why city energy managers and universities can have such a positive effect on city climate action.