Using two million census records, we document cultural assimilation during the Age of Mass Migration, a formative period in US history. Immigrants chose less foreign names for children as they spent more time in the US, eventually closing half of the gap with natives. Many immigrants also intermarried and learned English. Name-based assimilation was similar by literacy status, and faster for immigrants who were more culturally distant from natives. Cultural assimilation affected the next generation. Within households, brothers with more foreign names completed fewer years of schooling, faced higher unemployment, earned less and were more likely to marry foreign-born spouses.