Duff was born in Charleston, part of Bremerton, Washington.
He graduated in 1932 from Roosevelt High Schoolin Seattle, where he began acting in school plays after he was cut from the school basketball team.
Thereafter, he worked locally in the theater in Seattle until he entered the military during the Second World War where he was assigned to the Army Air Force's radio service.
After the war, he played Sam Spade in radio shows for ABC, CBS and NBC, from 1946 to 1950. He made his first film role as an inmate in "Brute Force" (1947), followed by "The Naked City" (1948). His other film credits include "Calamity Jane and Sam Bass" (1949), "Panic in the City" (1968), "In Search of America" (1971), "Kramer vs. Kramer" (1979) and "No Way Out" (1987).
Duff appeared in a number of films with his first wife, actress/director Ida Lupino. One of his later performances was as Dustin Hoffman's attorney in the Academy Award-winning Kramer vs. Kramer (1979).
On radio, Duff played Dashiell Hammett's private eye Sam Spade from 1946 to 1950, starring in The Adventures of Sam Spade on three different networks - ABC, CBS, and NBC. In 1951 Steve Dunne took over the role of Sam Spade.
Duff also appeared on television, in a 1955 episode of Climax! Entitled "Escape From Fear", From January 1957 to September 1958 "Mr. Adams and Eve" with his then-wife Lupino, He played the young Samuel Langhorne Clemens in "Bonanza"(1959), "The Twilight Zone"(1960), "Felony Squad", "Knott's Landing" and "Dallas".
From October 1960 through April 1961, Duff played Willie Dante, owner of the fictional San Francisco nightclub, Dante's Inferno, in the NBC adventure/drama series Dante.
In 1964, Duff guest-starred as Harold Baker in the episode Prodigy of NBC's medical drama about psychiatry, The Eleventh Hour.
He was then cast as Joe Stillman in the 1965 episode "Mountains to Climb" of the NBC education drama series, Mr. Novak.
From September 1966 through January 1969, Duff portrayed Detective Sergeant Sam Stone in the ABC police drama Felony Squad with costar Dennis Cole.
In 1977, he guest-starred in the Rockford Files episode.
In the 1980s, he appeared on NBC's Flamingo Road; and Knots Landing and Dallas.
Duff died at age seventy-six of a heart attack in 1990 in Santa Barbara, California.

Source: Wikipedia.org