Ethnohistoric documents characterize the thirteenth and fourteenth centuries in central Mexico as an era of endemic warfare and mass migration, processes that archaeologists have causally related to the development of the Aztec Empire. In this paper, we explore a striking transition in this dynamic, but poorly understood period at the Otomi capital of Xaltocan. The archaeological record reveals that with the adoption of Aztec II Black-on-Orange pottery by a.d. 1240, Xaltocan witnessed the expansion of the island to accommodate more residents, construction of the chinampa agricultural system, and the emergence of two subpopulations with distinct household organization, consumption, and funerary practices. We link a microscale examination of domestic activities with contextual understanding of macrolevel population dynamics, ethnic politics, and political-economic processes, and argue that this shift is due to an influx of migrants. These findings have significant implications for shifting conceptions of identity associated with the emergence of large polities.