He was born in Guilford, Connecticut and attended the Common schools. He was graduated from Guilford Academy in 1863 and from Yale College in 1867. He moved to Royalton, Vermont, in 1867 and became an instructor in the Royalton Academy. He studied law. He was admitted to the bar in 1870 and commenced practice in Bridgeport, Connecticut.
He served as prosecuting attorney for Bridgeport in 1872. He served as judge of the court of Common pleas for Fairfield County in 1874-1877.
He served as member of the State house of representatives in 1880.
De Forest was again elected mayor in 1889 and 1890.
De Forest was elected to the Fifty-second and Fifty-third Congresses (March 4, 1891 – March 3, 1895). He served as chairman of the Committee on Reform in the Civil Service (Fifty-third Congress). He was an unsuccessful candidate for reelection in 1894 to the Fifty-fourth Congress.
He served two terms as judge of the Common pleas court. He resumed the practice of law in Bridgeport, Connecticut, where he died October 1, 1924.
