Gerard Kenneth Tierney was born in Brooklyn, New York to Lawrence and Maria Tierney; his father was chief of the New York aqueduct police force. His older and younger brothers were fellow actors Lawrence and Edward Tierney, respectively. Edward subsequently left acting to become a building contractor, and Scott financed some of his projects.
Brady was reared in suburban Westchester County, New York. He was nicknamed "Jerry" in his youth. He attended Roosevelt and St. Michael's high schools where he lettered in basketball, football, and track. He aspired to become a football coach or a radio announcer but instead enlisted in the United States Navy before his graduation from high school. During World War II, he served in the United States Navy as a naval aviation mechanic overseas on the USS Norton Sound.
Discharged in 1946, Brady headed to Los Angeles, California. where his older brother Lawrence was already making some progress as an actor. First taking menial jobs as a cab driver and a lumberjack, Brady enrolled at the Bliss-Hayden drama school under his G.I. Bill of Rights. There he studied acting and did vocal training to eliminate his thick Brooklyn accent.
Brady had two brushes with scandal. In 1957, he was arrested for narcotics possession, but charges were dropped and he always maintained that he was framed.
In 1963, he was barred by the New York State Harness Racing Commission from participation in the sport due to his association with known bookmakers.
Brady specialized in tough-guy roles in films like He Walked by Night, Canon City, and Johnny Guitar, as the "Dancin' Kid".
From 1953 to 1956, Brady appeared four times in different roles on the anthology series, Lux Video Theatre. In 1955, he portrayed Ted Slater in "Man in the Ring" of NBC's anthology series, The Loretta Young Show. From 1953 to 1956, he appeared five times on the NBC anthology series, The Ford Television Theatre. In 1955 and 1957, Brady was twice cast on another anthology program, Studio 57. Early in 1957, he was cast in "The Barbed Wire Preacher" of the religion anthology series, Crossroads.
On December 26, 1957, he played the frontier figure William Bent in the episode "Lone Woman" of CBS's anthology, Playhouse 90, with Raymond Burr cast as his brother, Charles Bent. The plot involved the establishment in Bent's Old Fort on the Arkansas River in southeastern Colorado. Kathryn Grayson played the "Lone Woman," a Cheyenne Indian.
From 1955 to 1959, Brady appeared five times on CBS's anthology series Schlitz Playhouse, including as the roles of Reno Cromwell in "Night of the Big Swamp" and Calvin Penny in "Papa Said No." The Schlitz Playhouse episode "The Salted Mine" became the pilot for Brady's own western television series, Shotgun Slade, which aired seventy-eight episodes in syndication from 1959 to 1961.
In addition to Shotgun Slade, Brady appeared in several other television westerns, including Dick Powell's Zane Grey Theater, The High Chaparral, Lancer, Dirty Sally, The Virginian (twice), and Gunsmoke (three times).
In 1958, he played the lead role of Sergeant Matt Blake to Clint Eastwood's third billing as Keith Williams in Ambush at Cimarron Pass, a film which Eastwood is quoted as having called "probably the lousiest western ever made." This film is also notable for a scene in which Brady beats Eastwood in a fistfight, possibly the only screen-fight that Eastwood ever lost.
In 1961, he played the roles of John Keller in "We're Holding Your Son" on the anthology series hosted by Ronald W. Reagan, General Electric Theater, and Ernie Taggart in "Voyage into Fear" of the CBS detective series, Checkmate. In 1962, Brady was cast in the lead guest role as reporter/commentator Floyd Gibbons in "The Floyd Gibbons Story" of ABC's The Untouchables, starring Robert Stack. The next year he portrayed Bill Floyd in the episode "Run for Doom" of CBS's The Alfred Hitchcock Hour. In 1967, Brady guest starred on Carl Betz's ABC legal drama, Judd, for the Defense. In 1969, he portrayed Budd Blake in the episode "Panic" of the NBC drama Bracken's World. In 1973, he was cast as Davey Collier in "No Stone Unturned" of NBC's Banacek. From 1975 to 1977, Brady had the recurring role of "Vinnie" in sixteen episodes of NBC's Police Story crime drama.
On February 15, 1977, he appeared as Shirley Feeney's father, Jack Feeney, in the episode "Buddy, Can You Spare a Father?" on ABC's Laverne & Shirley. Though he had turned down the role of Archie Bunker on CBS's long-running show All in the Family, Brady appeared as Joe Foley on four episodes in 1976. He appeared five times on the James Garner NBC series, The Rockford Files. In 1977, he portrayed Lou Caruso in "Caruso's Way" of ABC's sitcom Welcome Back, Kotter, and appeared as Matt Zaleski in the TV miniseries Wheels the following year.
In 1983, Brady portrayed Alex Kidd in "Shadow of Sam Penny" on the CBS detective series Simon and Simon. Brady's last film acting role was as Sheriff Frank in the 1984 film Gremlins.
Brady had been involved earlier in life with actresses Gwen Verdon and Dorothy Malone but went on to marry a non-actress at age 43 and have two sons. A staunch supporter of Notre Dame Fighting Irish football, Brady hosted the team with a party at his home in the Hollywood Hills whenever the Irish came to town to play the University of Southern California.
In 1981, Brady was stricken with pulmonary fibrosis and thereafter required the use of an oxygen tank. He died four years later at the age of sixty. He is interred at Holy Cross Cemetery in Culver City, California.