MacLeod's acting career started in earnest in 1957, when he was 26. He appeared in guest roles on a number of TV shows and movies, including "Operation Petticoat," which starred Cary Grant and Tony Curtis, "The Party," which starred Peter Sellers, and "The Dick Van Dyke Show," which was the first time he worked with Mary Tyler Moore, though not the last. He also showed up on famous series such as "Perry Mason," "Hawaii Five-O," "The Man From U.N.C.L.E.," "Hogan's Heroes," and "The Andy Griffith Show" throughout the '60s.
In 1962, MacLeod landed his first regular series role on "McHale's Navy" as Joseph Haines, better known as Happy. The show was a sitcom in which MacLeod starred alongside Ernest Borgnine. He appeared in two seasons of the show and reprised the role for the two feature films that emerged from the series, "McHale's Navy" in 1964 and "McHale's Navy Joins the Air Force" in 1965.
In 1970, MacLeod found the first of two iconic roles of his career, joining "The Mary Tyler Moore Show" alongside cast members Mary Tyler Moore, Ed Asner, Valerie Harper, Cloris Leachman, and Betty White, the latter of whom joined the show in Season 4. MacLeod played Murray Slaughter, the hapless head writer of the Six O'Clock News who was frequently frustrated by news anchor Ted Knight and became a good friend to lead character Mary over the course of the series. As Murray Slaughter, MacLeod appeared in all 7 seasons of the show, even reprising the character for one episode of the spinoff series "Rhoda." MacLeod also received two Golden Globe nominations for Best Supporting Actor for his work on the show.
