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.
