This working paper examines the ideal role of city governments under a vertically integrated climate governance system designed to maximize urban mitigation potential. Action by city governments is essential for achieving deep reductions in global greenhouse gas (GHG) emissions. While many cities are already engaged in pioneering efforts to achieve such reductions, greater support from national governments could help realize urban mitigation potential more fully, quickly, and cost effectively. With greater policy coordination, cities could focus on roles and actions for which they are highly capable and best positioned. The authors find that under a coordinated approach designed to achieve deep GHG reductions, for roughly 20% of urban GHG abatement potential, cities’ ideal role is to be policy leaders and architects. The greatest opportunities here are in the passenger transport sector, and include improved spatial planning, promotion of walking and bicycling, enhanced transit system development, and more efficient transportation management.