A figure of the New Hollywood movement,[1] Forrest was best known for his collaborations with director Francis Ford Coppola, playing prominent roles in The Conversation (1974), Apocalypse Now (1979), One from the Heart (1982), and Tucker: The Man and His Dream (1988).

Forrest came to public attention for his performance in When the Legends Die (1972), which earned him a nomination for the Golden Globe Award for Most Promising Newcomer.

His other film credits include The Missouri Breaks (1976), Hammett (1982), Valley Girl (1983), The Two Jakes (1990), Falling Down (1993), and All the King's Men (2006), along with the television series 21 Jump Street, Lonesome Dove, and Die Kinder.

[6] Forrest was married three times: to his college girlfriend Nancy Whitaker from 1960 to 1963, to actress Marilu Henner from 1980 to 1983, and lastly to model Nina Dean in 1985.

McSpadden in the U.S. Civil War-themed movie Andersonville and real-life U.S. Army General Earle Wheeler in 2002's Path to War, the final film of director John Frankenheimer.