He engaged in farming and became member of the Minnesota Agriculture Administration Committee, serving from 1937 to 1941. He then served as state director of the Farm Security Administration (later the Farmers Home Administration) from 1941 to 1948.
Marshall was elected as a DFL member to the 81st, 82nd, 83rd, 84th, 85th, 86th, and 87th congresses, (January 3, 1949 – January 3, 1963); was not a candidate for reelection in 1962 to the 88th congress.
After Congress
After leaving Congress, he resumed agriculture pursuits, becoming a member of the National Commission on Food Marketing, and the United States Department of Agriculture Forest Appeals Board.