Testament has since released twelve more studio albums, and toured consistently, including sharing the stage with notable acts such as Iron Maiden, Judas Priest, Black Sabbath, Lamb of God, Devin Townsend, Kiss, Megadeth, Slayer, Anthrax, Exodus, Overkill, Korn, Sepultura, Suicidal Tendencies, Pantera, White Zombie, Primus, D.R.I.

[9] Billy's influences reportedly include John Lennon, Paul Stanley, Rob Halford, Ronnie James Dio, Bruce Dickinson, Phil Mogg, James Hetfield, Ozzy Osbourne, Geddy Lee, Bon Scott, Steven Tyler, Alice Cooper, Robert Plant, Phil Lynott and Klaus Meine.

Prior to Legacy and Testament, Billy was the vocalist for a local metal band called Guilt which also featured future Hericane Alice and Medicine Wheel guitarist Danny Gill.

In 2006, Billy provided vocals on the Sadus song "Crazy" and, along with several other musicians from other bands, did a cover of Iron Maiden's "Fear of the Dark" for their Numbers from the Beast tribute album.

In 2008, Billy did the vocals for Silent Night on the album of Christmas carols We Wish You a Metal Xmas and a Headbanging New Year with the likes of Scott Ian, Jon Donais, Chris Wyse and John Tempesta.

In 2015, Billy appeared on Metal Allegiance's self-titled album and provided featured vocals on the track "Can't Kill the Devil.

"[12] Billy and Jake Oni contributed vocals to the song "The Never" from Lamb of God guitarist Mark Morton's debut solo album Anesthetic, released in March 2019.

[15] In an August 2021 interview on The Jasta Show, Billy revealed that he might release a solo album in the future and hinted that its musical direction would be different from the traditional thrash metal style of Testament.

He is also recognized in the National Museum of the American Indian exhibit in the Smithsonian Institution titled "Up Where We Belong: Native Musicians in Popular Culture", which was on display through January 2, 2011.