Important phenomena are emerging that increasingly call for enhanced or more coordinated climate governance, but that seriously challenge the capacities of a traditional governance system centered on nation-states, their legal subdivisions, and intergovernmental organizations. Climate change represents one such governance challenge. In this Perspective, we ask: What role are sub-national actors poised to play in terms of accelerating a transition to deep decarbonisation? What sorts of mitigation contributions can such actors make, in lieu of or even in concert with national pledges under the Paris Accord? To answer these questions, this Perspective first defines and identifies a range of sub-national actions and efforts undergoing on climate change. It then reviews studies that quantify non-state emissions potentials, mentions limitations with the analysis, and concludes with implications for both policy and governance efforts. The potential of sub-national action for climate mitigation appears to be substantial—up to about 20 GtCO2e, far greater than existing NDC contributions—although prone to potential overlaps and uncertainties.