Mark's first role was at the age of ten as "Flip Rhinelander" in the 1959 film The Journey, starring Yul Brynner and Deborah Kerr. That same year, he appeared as Robbie Adams in "Another Day Another Dollar" of the NBC anthology series Alcoa Theatre. In 1960, he played 11-year-old George MacKay in the Doris Day and David Niven film, Please Don't Eat the Daisies. In the 1959-1960, television season, Mark appeared five times with Jon Provost as play-mate "Flip Rogers" on the CBS series, Lassie in episodes entitled "The Whopper", "Alias Jack and Joe", "Champ", "The Alligator", and "The Wallaby".
In 1960, Mark appeared as "Junior" in the episode "My Brother, the Hero" of the NBC sitcom, The Tab Hunter Show, starring Tab Hunter. In 1961, Mark appeared as "Dennis" in the episode "A Friend to Man" of the syndicated television series The Brothers Brannagan, with Stephen Dunne and Mark Roberts. He also appeared in 1961 as "Tommy" in the episode "Jack at Supermarket" of CBS's The Jack Benny Program.
Mark was also cast as Brook Hooten on Guestward Ho!, a 1960-1961 ABC sitcom starring Joanne Dru and Mark Miller, as a New York City family who relocates to rural New Mexico to operate a dude ranch. Thereafter, Mark appeared in guest shots in four popular CBS series, Have Gun - Will Travel, starring Richard Boone, The Andy Griffith Show, General Electric Theater, and My Favorite Martian. In the 1962-1963 season, he had a regular role as Larry Walker in CBS's Fair Exchange, the story of families in Great Britain and the United States who swap teenaged daughters for a year. Eddie Foy, Jr., played his father on the short-lived series, which also co-starred Lynn Loring and Judy Carne.
In 1964, Mark appeared as 15-year-old Kenny Hallop in the episode "Taps for a Dead War" of ABC's drama The Fugitive starring David Janssen. In the story line, Kenny is the son of Joe Hallop, played by Tim O'Connor, a man who had saved the life of the fictitious Richard Kimble during World War II but had himself been disfigured in the process. The same year, Mark appeared as Kenny Benjamin in the episode "The Special One" of the ABC science fiction series The Outer Limits. His episode co-stars were Macdonald Carey and Marion Ross.
From 1964-1969, Mark guest starred in several sitcoms, The Lucy Show in the episode "Lucy and the Missing Stamp" and Mister Ed on CBS, The Patty Duke Show on ABC, and The Mothers-in-Law on NBC. He then appeared as Fitzgibbons in the 1966 episode "The State v. Chip Douglas" of CBS's My Three Sons, starring Don Grady and Fred MacMurray.
From 1965-1966, Mark appeared as the first Steven Olson in the NBC soap opera, Days of Our Lives. In 1968, he guest starred as Jerry Frye in "The Good Thieves" of ABC's The Big Valley, starring Barbara Stanwyck. Mark's final screen roles were on CBS's Mission: Impossible as a delivery man (uncredited) and in the episode "The Bullet" of ABC's The Streets of San Francisco starring Karl Malden.