He had an unsettled childhood, his family never staying in one place for very long. The youngest of three brothers, Joel Anthony Fabiani was born to an Italian-Austrian father (Ernest Fabiani, 1902-1988) and an Irish and Native American mother (Bessie Fabiani, née Holcomb, 1903-1992). He attended seventeen different schools, eventually joining the army and then majoring in English at Santa Rosa Junior College. There, Joel was bitten by the acting bug and this led to a two year-long stint at the Actor's Workshop in San Francisco where he learned stagecraft. From there, he moved to New York and began his professional career in summer stock and off-Broadway. One of his first roles was as a court's clerk in the play One Way Pendulum at the now defunct East 74th Street Theatre. During this performance he met his second wife, the actress Audree Rae (his first marriage to fellow work shop alumnus Katharine Ross had ended in divorce after two and a half years in 1962). Joel made his debut Broadway appearance in a revival of Beyond the Fringe '65.

On screen from 1964, he featured in the pilot episode for Ironside (1967) and had a small recurring part in the medical soap The Doctors (1963). He also made a series of cigarette commercials as a debonair tuxedo-wearing James Bond-type character. This seemed to fit the bill for ITC producer Monty Berman who was scouting in the U.S. for an American actor to star in a projected espionage/crime series filmed in Britain. Before long, Joel and his wife relocated to the U.K. where the actor went on to make his name in a career defining role. As Stewart Sullivan, he was cast as the leader of a three-member team comprising the fictional Interpol sub-branch Department S (1969), solving the most enigmatic and perplexing cases, alongside Peter Wyngarde (as flamboyant crime writer Jason King) and Rosemary Nicols (as down-to-earth computer expert Annabelle Hurst). Joel's character, an ex-FBI agent, was most adept at handling himself in a fight and also the only member of the trio to interact directly and receive the team's assignments from the head of the department, a cultured diplomat with far-reaching contacts (played by Dennis Alaba Peters). Department S was a polished entertainment with global syndication and has long since attained cult status. Curiously, its brief run consisted of just one season and 28 episodes. A sequel, Jason King (1971), proved rather less successful.

Upon his return to the U.S., Joel (in his own words) "wanted to go out and conquer Hollywood, which is what I immediately set out to do - and didn't - but I had an awful lot of fun trying". Largely maintaining his suave Stewart Sullivan image, he went on to guest-star in numerous prime time shows (often for Quinn Martin Productions). In these, he played anything from murder victims to priests, from military brass to ill-fated spies, from attorneys and politicians to murderers and from private eyes to psychiatrists. Joel had recurring roles in the soaps Dallas (1978) (publisher Alex Ward), and Hanedan (1981) (the scheming King Galen of Moldavia), plus a decade-long tenure on All My Children (1970) (lawyer Barry Shire). Though occasionally seen in mainstream cinematic productions (Reuben, Reuben (1983), Tune in Tomorrow... (1990), Yılan Gözler (1998)), he had higher profile roles in made-for-TV films (the best of these being, arguably, the suspense thriller One of My Wives Is Missing (1976) and the Emmy Award-winning prison drama Attica (1980), co-starring Henry Darrow and Morgan Freeman).

For much of the 1970s, Joel continued to act on the New York stage. He also made TV and radio commercials for Sony. From the mid-80s, he diversified into the field of narration for audio books ("A River Runs Through It", "The Light in the Forest", "Aces and Eights") and worked as announcer on Barbara Walters specials until 2003. He retired from acting a decade later and resides in New York with his current wife Charna Greenburg.