Andrew Alcott Hallett was an only child. He was 6 foot 2 inches tall and hailed from the Cape Cod village of Osterville, Massachusetts, which is part of the town of Barnstable. He was a 1993 graduate from Barnstable High School and he went on to Assumption College in Worcester after graduation.
After moving to Los Angeles, Hallett worked as a Runner for an agency and then as a property manager. He eventually became a personal assistant to Kai Cole, wife of Buffy the Vampire Slayer creator Joss Whedon. When the couple and their friends saw Hallett singing at the "B.B. Kings" lounge at Universal CityWalk, Whedon conceived a new character for his new show Angel: Lorne, an anagogic demon who reads the hearts and futures of his guests as they sing in his karaoke bar. Hallett was invited to try out for the part, and won it after three auditions. Angel was his first real job as an actor, though he did play an extra role in a 1999 episode of Buffy; he acquired agent Pat Brady only after she spotted him singing and acting in "Judgment", the premiere episode of Angel's second season.
According to a 2005 interview, about a month after filming the last episode of Angel, Hallett suffered a dental infection which spread through the bloodstream to his heart, leading to a case of cardiomyopathy, for which he spent five days in the hospital. Although he recovered, his heart muscle and valves were weakened and he found himself easily fatigued afterward. He did not return to acting (except for a voice part in the animated film Geppetto's Secret), but pursued his music career and frequently appeared at media conventions for Buffy and Angel and for science fiction and fantasy in general.
Five years after first experiencing heart problems, and after at least three additional hospitalizations, Hallett died from congestive heart failure on March 29, 2009 at Cedars-Sinai Hospital in Los Angeles with his father by his bedside. He was 33 years old.