The United States absorbed nearly 22 million immigrants from Europe between 1880 and 1915. How did these immigrants, largely from undemocratic European states, become integrated into the American political system? This paper uses a newly assembled dataset of urban populations in the United States prior to World War I to investigate the decision of newly arrived immigrants to mobilize politically, focusing on the citizenship choice of foreign-born individuals in city wards. I find that immigrants were more likely to become politically active as their ethnic group’s share of the electorate grew, particularly in wards where the Democratic Party likely needed the vote of new immigrants to win elections.