He would appear in a number of western films, beginning with an uncredited role in Red Skies of Montana (1952) opposite Richard Widmark, then later more prominently as a gunslinger who tries to romance Claudette Colbert in 1955's Texas Lady.

Born Bernard Matt Ox in Wendell, North Carolina, Walcott was raised in Wilson, North Carolina. Walcott served in the United States Army towards the end of World War II and the Korean War. While serving in the United States Army, he appeared as a drill instructor in the film Battle Cry (1955), then as a military policeman in 1955's war-themed classic Mister Roberts with Henry Fonda, as the drill instructor with Tony Curtis in The Outsider (1961), and later Midway (1976) as Capt. Elliott Buckmaster.

Walcott had roles in many television series, including that of Stone Kenyon in two episodes of the NBC sitcom, The People's Choice with Jackie Cooper. He was frequently cast in westerns like Bonanza (seven times), Maverick, Frontier Doctor, Wagon Train, The High Chaparral, 26 Men, Sugarfoot (with Will Hutchins and cast opposite another guest star, Joi Lansing, in the 1958 episode "Bullet Proof"), Laramie, The Rifleman, The Tall Man, The Dakotas, and in several episodes of CBS's Rawhide, through which he began a long collaboration with Clint Eastwood. Walcott had featured roles in Eastwood's films Joe Kidd (1972), Thunderbolt and Lightfoot (1974), The Eiger Sanction (1975), and Every Which Way But Loose (1978).

Walcott made a guest appearance on Perry Mason as Bill Johnson in the 1959 episode, "The Case of the Howling Dog." He also was one of the stars of a 1961–1962 NBC television series, 87th Precinct, as Detective Roger Havilland. Walcott accepted guest roles on many popular television series, such as CBS's Dennis the Menace, with Jay North. He had recurring roles too in the original Dallas, Murder, She Wrote, and he appeared as Captain Diggs on the 1970s series Land of the Lost. He also made a guest appearance in 1984 on the TV series "Alice" in the episode titled "House Full of Hunnicutts". He played Jolene Hunnicutt's father, Big Jake Hunnicutt.

His theatrical film work included the comedy On the Double (1961) alongside Danny Kaye, the 1963 Gregory Peck film Captain Newman, M.D., Prime Cut (1972) with Lee Marvin, The Last American Hero (1973) starring Jeff Bridges, and the chase film The Sugarland Express (1974), directed by a 27-year-old Steven Spielberg. Walcott played a sheriff in the 1979 film Norma Rae, the film that won an Oscar for star Sally Field, and appeared in the Brooke Shields film Tilt the same year. He made a cameo appearance in the 1994 Ed Wood bio-pic starring Johnny Depp, directed by Tim Burton, which was Walcott's final role.

Walcott long regretted having anything to do with Plan 9, but in a September 10, 2000 Los Angeles Times interview, he said, "It's better to be remembered for something than for nothing, don't you think?" The movie was mentioned in two episodes of Seinfeld.

Walcott died of natural causes on March 20, 2015 in his home in Canoga Park, California, aged 87. He was laid to rest in Oakwood Memorial Park Cemetery in Chatsworth, California, beside his wife under his given name Bernard Matt Ox. He was also a devout Presbyterian and was a member of the Hollywood Presbyterian Church.