 From 1934 to 1942, he taught classes in Far Eastern and Latin American history, and also lectured some years on Greek and Roman history.He also attended the University of California at Los Angeles (1936–1937).In 1940, Mansfield ran for the Democratic nomination for the U.S. House of Representatives in Montana's 1st congressional district but was defeated by Jerry J. O'Connell, a former holder of the seat, in the primary; the general election was won by Republican Jeannette Rankin, who had previously won what was formerly an at-large seat in the House in 1916 and served until her defeat in 1920.