He began his aviation career during World War I as a fighter ace. He enlisted in the USAAS in Paris, underwent training, and reported to the 95th Aeronautical Squadron in February 1918. Sewall then became a balloon buster, shooting down an observation balloon each on 4 and 5 November.
After the war, he worked in a variety of jobs, including being an executive with Colonial Air Transport and a director of United Air Lincolnshire.
His political career began when he became an alderman in Bath in 1933. He was elected to the Maine state legislature as a representative in 1934, then as a senator in 1936 and 1938.
After the latter election, he was named President of the State Senate. In 1940, he was elected governor, and served two terms.
Sewall"s administration was notable for cleaning up scandals in state government and passing a minimum wage law for state teachers.
After stepping down as governor, Sewall became president of American Overseas Airlines for a year, then served as the military governor of Württemberg-Baden from 1946 to 1947. In 1948, Sewall finished a distant third in the Republican primary for Maine"s open United States Senate seat, which ended his political career. Sewall became president of the Bath National Bank in the 1960s.
He died on 26 January 1965.
Sewall married Helen Ellena Evans in 1929. Sewall"s grandfather, Arthur Sewall, was the Democratic candidate for vice president in 1896.