He was graduated from Guilford Academy in 1863 and from Yale College in 1867.
He served as the mayor of Bridgeport, Connecticut in 1878, and again in 1889. He also served in the Connecticut Senate and Connecticut House of Representatives. 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 mayor of Bridgeport in 1878.
He served in the State senate in 1882. He was Corporation counsel for Bridgeport.
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.
He was interred in Mountain Grove Cemetery.
He served as member of the State house of representatives in 1880.