This paper examines the implementation of carbon governance initiatives targeting urban buildings and energy infrastructure and uses a material politics approach to evaluate whether these practices are triggering trajectories towards decarbonization. Urban low carbon transitions suggest a substantial re-ordering of urban infrastructure. However, there is a critical need to engage with the material implications of low carbon practices since research so far has painted a picture of incremental ambitions struggling in implementation. This paper interrogates how carbon governance is implemented through urban buildings and energy systems, and the implications for urban decarbonization, by drawing on three urban case studies: Stockholm, London and San Francisco. The analysis draws on interviews with representatives from government, industry, utilities, building owners, and non-governmental organizations who are striving to achieve decarbonization in their cities. Patterns are emerging in what is being made to matter politically through the translation of carbon governance into building-energy infrastructure. In particular, the paper finds that (1) a short-term decision making timeline encourages action that incrementally reduces greenhouse gas emissions without fundamentally overcoming carbon lock-in, (2) actors are harnessing exceptional urban space to overcome the tyranny of cost-effectiveness in maintaining fossil fuel entrenchment (with concerning implications for justice and uneven development), (3) there is a pattern of individualization of responsibility for decarbonization, and (4) material politics are limiting the application of low carbon retrofits for the existing built form. Overall, this paper examines the implementation of urban carbon governance while encompassing the messy, materially embedded, and contested nature of infrastructure transformations.