His first television appearance was in 1956 in a production of The Adventures of Robin Hood (Episode "The Imposters"). He was a guest panelist in 1958, and again in 1963, on the CBS television game show What's My Line?
By 1965, he was back in Hollywood appearing in films and television shows for 20th Century Fox. He earned roles in the films Von Ryan's Express in 1965, Our Man Flint in 1966, and Caprice in 1967. He guest-starred in television programmes, including Daniel Boone and the Twelve O'Clock High episode "Siren Voices" as Luftwaffe Colonel Kurt Halland. In The Ghost & Mrs. Muir, a supernatural sitcom that ran from 1968 to 1970, he starred as Captain Daniel Gregg, and again was something of a successor to Rex Harrison, who had originated the role of Captain Gregg in the original 1947 film.
In 1969 Mulhare starred in Gidget Grows Up, an American made-for-television comedy film with Karen Valentine in the title role. He played Alex MacLaughlin, a love interest for Gidget. Harlan Carraher, who played Jonathan Muir in The Ghost and Mrs Muir with Mulhare, had a small part as well.
He also guest-starred in "Experiment In Terra", an episode of the original Battlestar Galactica.
Mulhare co-starred as Devon Miles, director of the Foundation for Law and Government, on Knight Rider (1982–1986) alongside David Hasselhoff's Michael Knight.
In the mid-1980s, Mulhare hosted the television series Secrets & Mysteries, also called Secrets of the Unknown, a magazine show that examined historical mysteries and the paranormal.
