In the absence of federal requirements, how do state- and municipal-level characteristics impact the probability of local policy innovation? This article provides insight by examining the adoption of sub-national climate change mitigation initiatives in the United States. Drawing from literature on policy innovation, a multilevel model is developed to examine the factors influencing over 900 U.S. cities to eschew free-rider tendencies and formally commit to greenhouse gas reduction. Multilevel analysis recognizes the nested structure of cities within states and accounts for the shared economic, political, and policy environments experienced by cities within the same state. The level of initiative state governments have taken toward climate protection varies considerably, and the influence of different state policies on related local decisions is empirically examined. Results are consistent with hypotheses derived from the innovation literature and suggest local-level characteristics are the dominant drivers of cities’ decisions to commit to climate protection.