In 1937, Mahler immigrated to America at the invitation of YIVO and settled in New York City. He worked as a lecturer in YIVO's research student courses, the Jewish National Labor Alliance's teachers seminary, and the Workmen's Circle's teachers courses. He was also a lecturer of the Herzliah Hebrew Teachers' Institute from 1938 to 1939, and in World War II he taught Jewish history at the Jefferson School for Social Studies and the School of Jewish Studies.[1] He wrote extensively on Jewish history, specializing in the socio-economic theory of Polish Jewry, the history of Jewish social and religious movements, and Jewish historiography.