Elites are dynamically emergent and evolving groups, yet their organization at any given time has tremendous implications for the tenor of social life and the probability of historical change. Using data on more than 3,000 Senatorial office-holders and over 3,100 elite marriages in early modern Poland, this article systematically documents changes over time in the structure of the Polish elite between 1500 and 1795 from a “multiple-networks” perspective. It measures timing of entry into senatorial ranks, regional integration of the elite, degree of elite dominance, and patterns of overlap between office-holding and marriage networks across four distinct eras in Polish history. Aggregate network patterns reveal a system in the eighteenth century characterized simultaneously by widening political access and increasing super-elite political control. Highlighting these patterns makes better sense of the Polish nobility's distinct cultural practices than do other historical sociological accounts and illuminates the structural basis for Poland's remarkable constitutional moment in the late eighteenth century.