Darren's role in the 1961 World War II film The Guns of Navarone was an attempt to break out of his teen image. He was the singing voice of Yogi Bear in the 1964 animated film, Hey There, It's Yogi Bear!, on the song "Ven-e, Ven-o, Ven-a". Prior to that, he was the singing and speaking voice of "Jimmy Darrock" on an episode of The Flintstones. He then achieved success co-starring as impulsive scientist and adventurer Tony Newman in the science fiction television series, The Time Tunnel (1966–1967).
In the 1970s Darren appeared as a celebrity panelist on Match Game.
Later Darren had a regular role as Officer James Corrigan on the television police drama T.J. Hooker from 1983–1986. Subsequently he worked as a director on many action-based television series, including Hunter, The A-Team, and Nowhere Man, as well as dramas such as Beverly Hills, 90210 and Melrose Place.
In 1998 he achieved renewed popularity as a singer through his appearances on Star Trek: Deep Space Nine in the role of holographic crooner and advice-giver Vic Fontaine; many of his performances on the show were re-recorded for the album This One's From the Heart (1999). The album showed Darren, a close friend of Frank Sinatra, comfortably singing in the Sinatra style; the 2001 follow-up Because of You showed similar inspiration from Tony Bennett.