After stints as a cowpuncher and railroad station hand, he studied at the University of Wyoming, where he also played basketball. Impulsively, he quit college in 1937 and decided to go straight to Hollywood to become a star.

Working as a carpenter and department store detective during his early days, he gained experience appearing in summer stock and in the early 1940s he was given the chance to work with the Shakespeare Festival in Carmel. There he played "Orsino" in "Twelfth Night", "Malcolm" in "Macbeth", "Horatio" in "Hamlet" and the ungainly title role of "Richard III".

On Broadway, Cochran appeared in Hickory Stick (1944).

Goldwyn then put him in Wonder Man (1945) a Danny Kaye movie co-starring Virginia Mayo and Vera-Ellen; Cochran played a gangster. Columbia used him in another Boston Blackie, Blackie's Rendezvous (1945), where he played a villain, and in The Gay Senorita (1945), with Jinx Falkenburg.

Goldwyn used Cochran in another Danny Kaye movie with Mayo and Vera-Ellen, The Kid from Brooklyn (1946). After United Artists borrowed him to play a gangster in The Chase (1946), Cochran appeared in his prestigious drama, The Best Years of Our Lives (1946), playing a man who has an affair with Virginia Mayo while her husband Dana Andrews was away at war. The movie was a huge critical and commercial success.

Cochran supported Groucho Marx in Copacabanca (1947) for United Artists. Goldwyn got him to play another gangster opposite Kaye and Mayo in A Song is Born (1948), directed by Howard Hawks. He made his TV debut in "Dinner at Antoine's" for The Philco-Goodyear Television Playhouse (1949) and followed this with "Tin Can Skipper" for NBC Presents (1949). He returned to Broadway for a short lived revival of Mae West's Diamond Lil, supporting West. This revived Hollywood's interest in him.

In 1949 Cochran went over to Warner Bros, where he played psychotic mobster James Cagney's power-hungry henchman, Big Ed Somers, in the gangster classic White Heat (1949). He appeared opposite Mayo. Warner Bros would eventually take over Cochran's and Mayo's contracts from Goldwyn.

Cochran supported Joan Crawford in The Damned Don't Cry (1950), then was given his first lead role in Highway 301 (1950), playing a gangster. He was a villain to Gary Cooper's hero in Dallas (1950), and played a Ku Klux Klan member in Storm Warning (1951), with Ginger Rogers and Doris Day.

Warners starred him in a war movie, The Tanks Are Coming (1951), and a Western, The Lion and the Horse (1952). He co-starred with Cornel Wilde in Operation Secret (1952) and supported Virginia Mayo in a musical, She's Back on Broadway (1953). In The Desert Song (1953), Cochran played Gordon Macrae's rival for Kathryn Grayson. He then left Warners.

Cochran was in trouble with the police a number of times in his life, including a reported assault and a charge of reckless driving in 1953.

Back in Hollywood he made Private Hell 36 (1954) with Ida Lupino for Director Don Siegel. He did "Foreign Affair" (1954) for Robert Montgomery Presents and "The Role of a Lover" (1954) and "The Most Contagious Game" (1955) for Studio One, "Trip Around the Block" (1954) and "The Menace of Hasty Heights" (1956) for The Ford Television Theatre, "The After House" (1954), "Fear is the Hunter" (1956), "Bait for the Tiger" (1957) for Climax!, and "The Seeds of Hate" (1955) for General Electric Theatre.

Republic Pictures hired him to play Ann Sheridan's love interest in Come Next Spring (1956). Cochran then went to England to play the lead in The Weapon (1956).

On TV he did "Outlaw's Boots" (1957) for Schlitz Playhouse, "Debt of Gratitude" (1958) for Zane Grey Theater, and "Strictly Personal" (1958) for The Loretta Young Show.

Cochran had the lead in an Allied Artists Western, Quantrill's Raiders (1958) and a Roger Corman gangster film, I Mobster (1959). Albert Zugsmith used him for the lead in The Beat Generation (1959) and The Big Operator (1959).

However from this point on Cochran worked mostly in television, guest starring in series such as 'Bonanza, The Untouchables, Route 66, Bus Stop, Stoney Burke, The Naked City, Shirley Temple's Storybook, The Dick Powell Theatre, The Virginian, Route 66, Death Valley Days, Mr. Broadway, Burke's Law and the 1959 episode "What You Need" (S1, Ep. 12; airdate: Dec. 25, 1959) of CBS's The Twilight Zone.

Cochran has a star at 1750 Hollywood Boulevard in the Motion Pictures section of the Hollywood Walk of Fame. It was dedicated February 8, 1960.

Cochran was Merle Oberon's co star in Of Love and Desire (1963), shot in Mexico. He had the lead in Mozambique (1964) for Harry Alan Towers.

On June 15, 1965, at the age of 48, Cochran died on his yacht off the coast of Guatemala, reportedly due to an acute lung infection. His body, along with three Mexican girls aged 14, 19 and 25, remained aboard for ten days since the girls did not know how to operate the boat. It drifted to shore in Port Champerico, Guatemala, and was found by authorities.

Cochran was a notorious womanizer and attracted tabloid attention for his tumultuous private life, which included well-documented affairs with numerous starlets and actresses. Mamie Van Doren later wrote about their sex life in graphic detail in her tell-all autobiography Playing the Field: My Story (New York: G.P. Putnam, 1987). He was also married and divorced three times, to Actress Fay McKenzie, Florence Lockwood and Jonna Jensen. Cochran was the grandfather of film and television Producer Alex Johns, who co-executive produced more than seventy episodes of the animated television series Futurama. In the 2002 documentary The Importance of Being Morrissey, Steven Morrissey claims that his parents named him after Steve Cochran.