Allan starred in the Lassie television series from 1968-1970 and has starred in many soap operas. He is perhaps best known for his role as Don Craig on Days of Our Lives, which he played from 1971 to 1985. He gained a new audience when he played C.C. Capwell on Santa Barbara from 1986 to 1993.
His departure from Days of Our Lives was very controversial, as his character was abruptly written out with no further explanation. In one episode, he went to mail a letter and never returned, and was never mentioned in dialogue again.
After his time on Days of Our Lives, Allan had a recurring role on Beverly Hills 90210 playing the role of Rush Sanders.
In 2004, he started playing the role of Edward Quartermaine on General Hospital. Critics of the casting move say that Allan is too young, with Stuart Damon, the actor who plays his son, being about a month older than Allan himself. In contrast, his wife on the show, Lila Quartermaine, played by Anna Lee, was 24 years older than he was. The age difference between Edward and Lila was of little issue as Lee was very sick by the time Allan played Edward, and so they never shared a scene together. Prior to starring on General Hospital he starred on its spinoff Port Charles as a head angel named Ed Grant.
In 2006, it was announced that John Ingle, the actor to play Edward before Allan, was returning, therefore booting Allan out of the role. Ingle began airing again in spring 2006.
Allan has also appeared in numerous movies made for television.
Summered at Nemarest, near Kent Cliffs, New York.