Cities play a vital role in the global economy, which necessitates incremental progress towards low-carbon growth to serve their long-term needs and remain competitive locally and collectively. Urban energy consumption makes the climate challenge more pronounced in cities, where simultaneously, the opportunities for mitigation are prevalent. From a climate governance perspective, this thesis examines the prioritization of climate action to meet greenhouse gas emission targets that come at a cost to cities and their residents. The thesis presents four papers that together structure climate action by developing tools to support cities in decarbonizing their economic growth. A three-pronged approach to climate action is developed to integrate key city documents:​ Greenhouse Gas Emissions Inventories, Climate Action Plans, and Budgets, and a review of practices and advancements in progressive cities are presented. Greenhouse gas inventories are the starting point of cities’ climate action, which are analyzed comparatively as they apply to community, corporate and upstream emissions, with examples from New York, Shanghai and Paris. Surprisingly not typically linked to inventories, climate action plans are strategic documents defining ambitious initiatives to reduce emissions. A methodology to compile mitigation measures suggested in climate policies and programs in a city, and that of higher orders of government is established, and their cost-effectiveness and abatement potential is calculated and illustrated in the form of a marginal abatement cost (MAC) curve. A case study of prioritizing mitigation measures in the energy supply, buildings, transportation and waste sectors is demonstrated for Toronto’s climate targets, drawing implications for the city’s 2020 and 2050 outlook. To integrate budgets with inventories and action plans, the METRO model is designed for cities to forecast greenhouse gas emissions and evaluate city budgets and business and household incomes relative to climate mitigation priorities. Applying the model to Toronto, the methodology of the METRO model is demonstrated to show that climate commitments have a quantitative dimension and an economic reality. Prioritized climate action positions cities for timely mitigation efforts, where knowledge sharing among cities is paramount to achieving low-carbon growth.