Policy interventions such as zoning ordinances, school facility siting guidelines, capital improvement programs, and park master plans hold particular promise for promoting physical activity, especially at the local level. Despite increasing attention to the relationship between built environment characteristics and physical activity, there is a paucity of research on the extent to which local policies can promote or hinder physical activity. Furthermore, the impact of local policies on physical activity should depend on how effectively the policies are implemented. Based on the policy implementation literature and using Montgomery County, Maryland, as a case study, this study identifies factors related to the successful implementation of local policies hypothesized to influence physical activity. For our study, we conducted an extensive policy review and 17 in-depth interviews with 26 individuals. The interviews were transcribed, coded, and analyzed to identify the relevant factors that affect policy implementation. Our findings suggest that knowledge and awareness, commitment and capacity, intergovernmental coordination, the presence of an advocate or champion, and conflict influence physical activity policy implementation at the local level. Those trying to increase physical activity through policy could focus on these implementation features to help make policy implementation more successful.