This research seeks to identify key influencers on a relatively new type of policy instrument, the Sustainable Purchasing Policy, in the local government context. Specifically, we analyze how vendor relationships and organizational capacity affect perceived Sustainable Purchasing Policy success. Using statistical analysis on a nationally distributed survey to city directors as well as interviews with city purchasing agents, we are able to identify what factors are likely to lead to successful policy implementation. Our findings show that cities benefit from their vendors providing reliable information regarding sustainable offerings, that vendors offering sustainable goods need superior technical capabilities to compete in a cost-driven environment, and that purchasing agents require support from a city’s top management if they want to successfully implement sustainable purchasing. Future avenues for research are discussed.