[2][3] Roberts was also known for his lifelong activism, which included participation in the Selma to Montgomery marches in 1965[2] and pressuring NBC to refrain from hiring White people to portray minority characters.

Later, he spent eight weeks at the Bryn Mawr College Theatre in Philadelphia, portraying Dan in Emlyn Williams' Night Must Fall and Alfred Doolittle in Bernard Shaw's Pygmalion.

[6] In 1952, Roberts moved to New York City, where he appeared first off-Broadway in one-act operas and ballets with the North American Lyric Theatre, with the Shakespearewrights, at the Equity Library Theatre, and later on Broadway with performances in Tonight in Samarkand (also in Washington, DC), The Lovers opposite Joanne Woodward, and A Clearing in the Woods with Robert Culp and Kim Stanley.

Roberts signed a contract with Columbia Pictures in 1957, and made his film debut a year later as one of Burl Ives' contentious sons in Desire Under the Elms (1958).

He also landed a character role in The Sheepman (1958), opposite Glenn Ford and Shirley MacLaine, and continued to guest-star on television shows such as episodes of Shirley Temple Storybook Theater ("The Emperor's New Clothes", "Rumplestiltskin", "The Sleeping Beauty", and "Hiawatha"), the live-broadcast Matinee Theater, where he starred again in Shakespeare's Much Ado About Nothing, and in The Heart's Desire.

In 1959, Roberts guest-starred in episodes of General Electric Theater, Cimarron City, Sugarfoot, Lawman, One Step Beyond, Bronco, 77 Sunset Strip, The Detectives, and House Call.

"If Roberts felt typecast by Westerns, they also provided his finest role in this film, arguably the greatest of the B-movies starring Randolph Scott and directed by Budd Boetticher.

Roberts recognized the film's classic structure; his engaging outlaw, Sam Boone, counterpoints Scott's granite-faced Ben Brigade, maintaining the tension of whether they will work together or clash.

Roberts played Ben Cartwright's urbane eldest son Adam, in the Western television series Bonanza.

[3] "It was perhaps not surprising that, despite enormous success, he left Bonanza after the 1964–65 season, criticizing the show's simple-minded content and lack of minority actors...".

[10] It particularly distressed him that his character, a man in his 30s, had to defer continually to the wishes of his widowed father,[11] and he reportedly disliked the series itself, calling it "junk" television[5] and accusing NBC of "perpetuating banality and contributing to the dehumanization of the industry.

"[5] The equally self-critical Roberts ("I guess I'll never be satisfied with my own work"[12]), "had long disdained the medium's commercialization of his craft and its mass-production, assembly-line mindset.

"[19] Finally, after disagreements with writers and producers over the quality of the scripts, characterization, and Bonanza's refusal to allow him to perform elsewhere while on contract, Roberts "turned his back on Hollywood wisdom and well-meant advice," and left, largely to return to legitimate theater.

[20][21] Roberts fulfilled, but did not extend his six-year contract for Bonanza, and when he left the series, his character was eliminated with the explanation that Adam had "moved away.

"[11] Later episodes suggested variously that Adam was "at sea", had moved to Europe, or was on the East Coast, running that end of the family business.

"[22] In the last two Bonanza movies that aired on NBC in the early 1990s, the story line stated that Adam, now in Australia, had equaled his father's success, dominating the engineering/construction business.

Roberts was the only accomplished singer of the original cast, though David Canary, who joined Bonanza in 1967, had a background in voice and performed on Broadway.

[24] On the Bonanza box-set albums, Roberts also sings "Early One Morning", "In the Pines", "The New Born King", "The Bold Soldier", "Mary Ann", "They Call the Wind Maria", "Sylvie", "Lily of the West", "The Water is Wide", "Rake and a Ramblin' Boy", "A Quiet Girl", "Shady Grove", "Alberta", and "Empty Pocket Blues".

"Mata Hari was a show with a great story, two fascinating characters, and some accessory mess that could have easily been tidied up by anyone but Vincente Minnelli."

[30] His additional stage credits after Bonanza include Two for the Seesaw, A Thousand Clowns, One Flew Over the Cuckoo's Nest, Any Wednesday, and The Sound of Music (as Captain von Trapp).

Roberts guest-starred in TV shows such as The Girl from U.N.C.L.E., The Virginian, The Big Valley, Lancer, Mission: Impossible (four episodes), Have Gun – Will Travel, Marcus Welby, M.D., The Wild Wild West, Ironside (two episodes), The Rockford Files, Gunsmoke, Mannix, Vega$, The Odd Couple, Hawaii Five-O, The Love Boat, Hotel, The Hardy Boys/Nancy Drew Mysteries, Nakia, Night Gallery, The Bold Ones, The Quest, Police Story, Most Wanted, Westside Medical, Man from Atlantis, Jigsaw John, Sixth Sense, Quincy, M.E.

He co-starred or was featured in several TV movies, including, The Adventures of Nick Carter, Dead Man on the Run, Assignment Vienna's pilot Assignment: Munich, The Night Rider, The Silent Gun, The Lives of Jenny Dolan, The Deadly Tower, Hot Rod, Desperado, The Bravos, and High Noon, Part II: The Return of Will Kane.

Roberts told TV Guide in 1979 that he chose to return to weekly television after watching his father age and realizing that it was a vulnerable time to be without financial security.

He guest-starred as Hezekiah Horn in the powerful Young Riders episode, "Requiem for a Hero", for which he won a Western Heritage Award in 1991.