This dissertation examines the Americanization of immigrants as a defining theme in American musical theater. It does so through studies of productions from across the past century about Irish Americans, Chinese Americans, and Latino/a Americans, and in each case, at least one of the creators is a member of the ethnic American group depicted. I contend that these artists found the musical to be a constructive tool for voicing their experiences of the struggle of Americanization and broadening notions of American identity. The resulting narrative expands upon the substantial "golden age"-centered literature on Jewish assimilation and the American musical. Decentralizing the "golden age," I show how the genre has helped write into cultural citizenship a broad range of immigrant groups during fraught periods in which their national belonging was contested. I draw upon a wide range of disciplines - especially immigration history, ethnic studies, and American studies as well as musicology - and diverse methods, including archival research, oral history, textual and musical analysis, reception history, and historically based hermeneutics.