 Between 1936 and 1938, Handlin taught history at Brooklyn College before reentering Harvard University.According to Handlin, it was Arthur M. Schlesinger, Sr. who "directed my attention to the subjects of social history that have since occupied much of my attention," urging Handlin to write his dissertation on immigration to Boston in the late 18th and early 19th centuries.In 1940 he received his PhD, joining the faculty in 1939 and remaining until 1986, his work centering around the topic of immigrants in the U.S. and their influence on culture.