This article links policy outputs in city climate action plans with environmental outcomes. This task is challenging because different human activities in cities vary in terms of their contributions to greenhouse gas (GHG) emissions and because the engineered infrastructures that support these activities extend well beyond the city scale. I present a generalizable quantitative approach that uses the transboundary infrastructure supply chain GHG emission footprints of cities to identify key actors and policy levers most effective in reducing the global GHG impact of cities. This infrastructure supply chain GHG emission footprint represents the life-cycle energy associated with provisioning key infrastructure services—water, energy, food, shelter, sanitation, mobility, connectivity, and public spaces—to support the activities of households, businesses, and industnes in cities.