DeWitt was born in Sully, Iowa, served in the Navy during World War I, and from 1921 to 1924 attended Drake University.
He married Marjorie Aileen Everett on October 3, 1927, and with her had a son and daughter.
DeWitt spent his working career with the Travelers Insurance Company, having joined in 1925 as a claims investigator, and then rising through the ranks until he became president (1952–1965), director, and ultimately chairman of the board.
He also served as a director of the Chase Manhattan Bank, the Hartford National Bank and Trust Company, Hartford Hospital, the Southern New England Telephone Company, and United Aircraft Corporation.
Today DeWitt is best remembered for his monographs on American political history, and for his extraordinary collection of American presidential campaign memorabilia, which contained more than 25,000 individual pieces, and which he donated to the University of Hartford.