The younger brother of child actor Darryl Hickman, he also began his work at an early age. One of his earliest roles was an appearance in the 1942 Our Gang comedy Melodies Old and New. He first gained wide notice when he played a recurring role as Bob's nephew on the situation comedy The Bob Cummings Show (a/k/a, Love That Bob) in 1955 while a student at Loyola University (now known as Loyola Marymount University). This role probably led as much as anything to his casting in the lead role in the CBS sitcom The Many Loves of Dobie Gillis. Interestingly, although at the show's debut the Dobie character was still a teenager in high school, Hickman was already 25.

After playing Dobie for four years (with fellow former Loyola student Bob Denver as his sidekick, Maynard), Hickman found himself stereotyped as a "youngster" just at the time of his life when he was too old to play any further such roles. He appeared in some minor "beach" films and made an unsuccessful TV pilot for a program in which he was to play a young schoolteacher.

In 1965 he also appeared in the comedy Cat Ballou along with Jane Fonda, Lee Marvin, and Nat King Cole. The film is notable for the fact that Lee Marvin won the Academy Award for Best Actor in a Leading Role and Nat King Cole died before the movie was released.

Hickman found his future in entertainment to be behind the scenes, and became involved in several projects in production roles. Later he became a programming executive at CBS, a role which he has since spoofed by appearing as such in several on-camera roles. He also worked as a director on various episodes of television series, and also portrayed Dobie in two TV reunions, Whatever Happened to Dobie Gillis and Bring Me the Head of Dobie Gillis. His autobiography is entitled Forever Dobie.