How do organizations obtain access to valued resources without diluting the loyalties and identities of their members? Network analysts suggest focusing on the boundary-spanning activities of "brokers" who bridge gaps in social structure. In many contexts, however, brokers are viewed with suspicion and distrust rather than rewarded for their diversity of interests. This dissertation examines organizations in which the theoretical deck is seemingly stacked against brokerage and toward parochialism: American-Italian mafia families. Through an institutional analysis of the mafia organization, I trace how ethnic and organizational closure led marginalized actors to seek alternative paths to enrichment beyond the family-controlled networks and industries. Using a historical network data set, I document a division of network labor in which a small number of brokers - often, surprisingly, ethnic outsiders and lower-status criminals - bridged otherwise disconnected islands of criminal activity. More than coordination among elite criminals, it was entrepreneurial action by marginal and excluded actors - outsiders operating largely beyond the control of mafia organizations themselves - that generated the integrated and highly connected mafia network. This dissertation accounts for a striking historical paradox by showing how it was possible for the American Mafia to appear for all intents and purposes to be a well-organized national conspiracy even as the individual groups involved remained organizationally and geographically separate from one another.