Hallet, best known for his recurring role as Lorne in hit US TV show Angel, died from heart failure following a five-year battle against heart disease.
He landed his first break after he was dragged on stage at a concert to sing with Patti LaBelle - the performance kickstarted his career as a singer-songwriter.
Hallett was spotted during a revue at a Los Angeles nightclub by Buffy The Vampire Slayer creator Joss Whedon, who created the demon character Lorne in the show's spin-off Angel just for him.
The star appeared in 76 episodes before series ended in 2004. Hallet's subsequent ill health ended his acting career.
He died on Sunday at Cedars-Sinai Hospital in LA, with his father Dave Hallett by his side.