Ethnic identities around the world are deeply linked to the modern territorial state, yet it is often unclear to what extent ethnicity shapes states or states shape ethnic identities. I argue that governments at the national and subnational level have incentives to bias governance in favor of the largest ethnic groups in their territory. The resulting disadvantages for ethnic minorities can motivate minority assimilation and emigration. Both reactions gradually align ethnic with administrative boundaries. I examine this process at the subnational level in 20 countries in Sub-Saharan Africa. Exploiting credibly exogenous, straight borders allows for causal identification. I find substantive increases in local population shares of administrative units’ predominant ethnic groups at the border, showing that administrative geographies shaped ethnic groups. Additional analyses demonstrate that ethnic assimilation and emigration of local minorities drive the phenomenon. These results highlight important effects of the territorial organization of modern governance on ethnic groups.