Although the dominant paradigm of immigrant employment views immigrants as clustered in a limited number of occupations or industries that comprise a niche, the explanations of how immigrants enter and establish these niches remain incomplete. While most researchers emphasize the importance of social networks, the social network approach begs the issue of how to account for the insertion and consolidation of immigrant networks as opposed to those dominated by incumbent native workers. This article seeks to answer this question through a case study of immigrant professional employees in New York City government. I argue that the growth of this immigrant niche resulted from changes in the relative supply of native workers and in the structure of employment, which opened the bureaucracy to immigrants and reduced native/immigrant competition. These shifts opened hiring portals; given the advantages of network hiring for workers and managers, and an immigrant propensity for government employment, network recruitment led to a rapid buildup in immigrant ranks.