Bonanza

He came to prominence playing Ben Cartwright's urbane eldest son, Adam, in the Western television series Bonanza. Despite the show’s success, he left after the sixth season in 1965 due to disagreements with the writers and a desire to return to legitimate theatre. Among other complaints, Roberts argued that a 34-year-old, educated, Eastern-born man would not be calling his father "Pa." The writers tacitly agreed not to exceed three "Pa" references per episode. According to producer David Dortort in the February 2006 "Bonanza Gold" issue, Roberts also wanted to stop wearing his toupee. Since, in real life, there were fewer than thirteen years of age between Roberts and Lorne Greene, a bald Adam would not have translated well on screen. Bonanza continued without Roberts for another eight seasons.

While performing in the series, Roberts recorded Come All Ye Fair and Tender Ladies, a folk music album which Allmusic calls "...the softer, lyrical side of folk music — pleasant and not challenging, but quite rewarding in its unassuming way.". The album has been released on RCA Victor LPM / LSP 2662 (1963), and was arranged by Dick Rosmini. The album presently is available on compact disc only as part of the fourth disc of the Bonanza 4-CD boxed set on Bear Family.

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". Roberts was the only trained/ accomplished singer of the original Ponderosa clan (David Canary, who joined the cast in 1967, graduated as a voice major) Lorne Greene's deep baritone voice scored big in songs like "Ringo" (1964). Blocker narrated intros and Landon held his own.

After Bonanza
Roberts continued to do guest shots on TV shows such as The Big Valley, Mission: Impossible, The Wild Wild West, Gunsmoke, Mannix, The Odd Couple, Hawaii Five-O, and The Hardy Boys. His rich baritone voice was displayed when he played Jigger in an ABC television presentation of Carousel and Rhett Butler in the Los Angeles stage production of Scarlett.

He regained star status in the early 1980s while starring in the television series Trapper John, M.D. (1979-86). Roberts played the character almost twice as long as Wayne Rogers did on the CBS M*A*S*H series (1972–1975).

In 1988, Roberts co-starred with Milla Jovovich in the TV movie The Night Train to Kathmandu. A guest appearance as Hezekiah Horn in the Young Riders episode "Requiem for a Hero" won a Western Heritage Award for Roberts in 1991.

Roberts hosted FBI: The Untold Stories in 1991.

He is now retired, the lone surviving original Bonanza cast member. Roberts made his last TV appearance (to date) on a 2001 Diagnosis Murder, updating a Mannix character he had portrayed decades before.

In the 1980s and '90s, playing off his Trapper John M.D. persona, Roberts was a TV spokesman for Ecotrin, a brand of analgesic tablets.