Slade began his career on the Broadway stage appearing in the Josh Logan-directed play, "There Was a Little Girl," Jane Fonda's first Broadway play. He then earned a role in the 1961 film "Splendor in the Grass", directed by Elia Kazan and filmed in upstate New York. In the early 1960s, he moved to the West Coast, where he was cast as Seaman Jimmy "Red" Smith in the feature film "Voyage to the Bottom of the Sea" (1961). Despite his character being killed in the movie version, Irwin Allen brought Slade back for the "Voyage to the Bottom of the Sea (TV series)" on ABC. He was cast in 1964 as a new character, Seaman Malone. He was hence the only one of six actors to have been cast in both the film and television versions of "Voyage to the Bottom of the Sea". He appeared only in the first half of the first season because he departed to become a semi-regular, Eddie, in the CBS sitcom, "Gomer Pyle, U.S.M.C.", starring Jim Nabors. Slade appeared in eight episodes of "Gomer Pyle", all of which were aired during the first half of the first season. He was cast in three episodes of the NBC education drama, "Mr. Novak", starring James Franciscus in the title role. His first television role was as Stu Walters in the 1961 episode "Deadline" of the ABC sitcom, "My Three Sons", starring Fred MacMurray. In 1963, he was cast in the episode "A Girl Named Amy" of Jack Lord's ABC series, "Stoney Burke", a rodeo adventure series. In 1964, he guest-starred in "The Enormous Fist" episode of "Rawhide" opposite Eric Fleming and Clint Eastwood. That year, he also appeared as the title character Michael Manning, alias Michael Da Vinci, in the "Perry Mason" episode, "The Case of the Careless Kidnapper".