He has appeared in films and television since the early 1960s. His movies include the title role in Ensign Pulver (1964) with Burl Ives and Walter Matthau, The War Wagon (1967) with John Wayne and Kirk Douglas, the title role in Young Billy Young alongside Robert Mitchum in 1969, Easy Rider, also in 1969, and Beware! The Blob, or—Son Of Blob in 1972. In 1982 he starred in Angkor: Cambodia Express with Nancy Kwan, Christopher George, Woody Strode, and Sorapong Chatree.

In the 1960s, Walker appeared in a 1962 episode ("Across Walnuts and Wine", S3E07) of the classic television series Route 66. He played the title role and an emotionally disturbed character who was a troubled actor who lived and performed on the streets and in circuses, in Naked City episode "Dust Devil on a Quiet Street" from November 28, 1962. In The Big Valley episode "My Son, My Son," aired on November 3, 1965 (S1E08), Walker portrayed Evan Miles, an emotionally disturbed college dropout who becomes obsessed with childhood friend Audra Barkley. He had a memorable role in Star Trek as Charles 'Charlie' Evans, a 17-year-old adolescent and social misfit with psychic powers in the episode "Charlie X", which aired September 15, 1966 (S1E02). Walker appeared in the fifth season of the series Combat! in the episode "Ollie Joe" on September 27, 1966 (S5E03). In addition he had the title role in "Billy the Kid," episode 22 of The Time Tunnel, which originally aired on February 10, 1967. He also portrayed Nick Baxter, an ill alien who caused the deaths of humans by touch, in the episode "Panic" in the television series The Invaders, which aired on April 11, 1967 (S1E14). He played Mark Cole in the October 29, 1967, episode of Bonanza titled "The Gentle Ones" (S09E07).

In the 1970s, Walker had a role in an episode of Columbo, "Mind Over Mayhem" (S3E06, 1974), and as an innocent longshoremen who takes the blame for a murder on Quincy, ME "The Hero Syndrome" (S3E09) in 1977. He also appeared in the 1978 pilot episode of The Eddie Capra Mysteries

In later years Walker maintained an episodic presence on TV, he guested in two episodes of Murder, She Wrote with Angela Lansbury: once in 1987 and in 1990 as a mentally handicapped man in "Shear Madness" (S06E20). His last screen performances were on the TV series L.A. Law and In the Heat of the Night, both in 1991. He also appeared in a TV series appearance in 1993, and a final film appearance in a small role in the 2016 film Beyond the Darkness.