[4] In 1990, Quinn did television commercials for Pepsi and Ray-Ban, appeared in the music video for the Richard Marx song "Satisfied", and had his first speaking line in the pilot of Beverly Hills, 90210 after having endured eight separate auditions for the roles of Brandon Walsh and Steve Sanders[4] (played by Jason Priestley and Ian Ziering, respectively).

Casting director Johanna Ray gave him a small role with two speaking lines in the pilot, however Quinn was barely visible in the final broadcast version.

In 1992, while still on Roseanne, Quinn also took a main role on the American and British TV series Covington Cross and in the same year, co-starred with Holly Marie Combs in the slasher film, Dr.

In 1999, after seven years of using an American accent on Roseanne, Quinn was pleased when producers cast him in the role of Allen Francis Doyle on the Buffy the Vampire Slayer spin-off series, Angel.

[2][4][8][9] He returned to Ireland in 1997 to be with his family, who supported him in achieving short periods of sobriety and spent time in a drug and alcohol rehabilitation center.

[10] On 3 December 2002, at the age of 32, Quinn was found dead at a friend's home in North Hollywood, California, where he had been staying for a few months.

"[13][14] In 2019, during Entertainment Weekly's 20th-anniversary reunion of the cast of Angel, actor David Boreanaz stated that "Glenn played a great character, but also became a really close friend of mine.