Despite the growing importance of cities in climate change mitigation, there remains a dearth of rigorous analysis to inform municipal climate policies and mitigation strategies. Our study addresses this gap in the literature. We develop an energy system optimization model for urban-scale decarbonization, and use it to analyze the Community Climate Plan adopted by Austin, Texas. Austin is a valuable testbed for analysis because it is a member of the C40 Cities Climate Leadership Group and its goal of net-zero greenhouse gas emissions by 2050 is particularly ambitious. We find that this policy objective can be achieved at a modest 2.7% increase in net present power and transportation costs relative to business-as-usual. The optimal decarbonization pathway proceeds through two distinct stages, first reducing power sector emissions, then electrifying transportation. Solar PV expands in the long run with or without the climate plan based on favorable cost projections, but the policy causes wind to replace natural gas as a complement to solar PV. Results also highlight the substantial value of intelligently scheduled battery storage operations and electric vehicle charging.