In the 1930s, with the rise of Nazism, many artists in Europe had to flee their homelands and sought refuge in the United States. Austrian composer Hanns Eisler who had risen to prominence as a significant composer during the Weimar era was among them. A Jew, an ardent Marxist and composer devoted to musical modernism, he had established himself as a writer of film music and Kampflieder, fighting songs, for the European workers' movement. After two visits of the United States in the mid-1930s, Eisler settled in America where he spent a decade (1938-1948), composed a considerable number of musical works, including important film scores, instrumental music and songs, and, in collaboration with Theodor W. Adorno, penned the influential treatise Composing for the Films. Yet despite his substantial contributions to American culture American scholarship on Eisler has remained sparse, perhaps due to his reputation as the "Karl Marx in Music." In this study I examine Eisler's American exile and argue that Eisler, through his roles as a musician and a teacher, actively sought to enrich American culture. I will present background for his exile years, a detailed overview of his American career as well as analyses and close readings of several of his American works, including three of his American film scores, Pete Roleum and His Cousins (1939), Hangmen Also Die (1943), and None But the Lonely Heart (1944), and the String Quartet (1940), Third Piano Sonata (1943), Woodbury Liederbüchlein (1941), and Hollywood Songbook (1942-7). This thesis builds upon unpublished correspondence and documents available only in special collections at the University of Southern California (USC), as well as film scores in archives at USC and the University of California, Los Angeles. It also draws on Eisler studies by such European scholars as Albrecht Betz, Jürgen Schebera, and Horst Weber, as well as on research of film music scholars Sally Bick and Claudia Gorbman. As there is little written on the particulars of Eisler's American years, this thesis presents new facts and new perspectives and aims at a better understanding of the artistic achievements of this composer.