The structure of social networks within and across cities influences urban creativity and the potential for new knowledge creation. We propose that cities achieve greater inventive creativity when there is (1) a short average social distance among inventors in a city, and (2) those inventors are highly clustered. A short average social distance allows knowledge to circulate quickly across a larger pool of knowledge sources; clustering promotes trust, cooperation, and a more effective use of the acquired knowledge. At the same time, a short average social distance between a city’s inventors and knowledge sources outside the city will enrich and renew a city’s inventive network by facilitating faster access to fresh external knowledge. We find evidence to support these propositions in a longitudinal study of the inventive productivity of 331 U.S. cities