Before singing for Exodus, he was the lead singer for the band Legacy (which became Testament after his departure), recording and writing most of The Legacy demo. At the time of his departure of Legacy, he introduced Chuck Billy to Legacy members and joined Exodus.
Souza is best known for his stints in Exodus, for whom he sang between 1986 and 1994 (when they broke up) after being brought in to replace Paul Baloff. Exodus was disbanded about two years after the release of Force of Habit. He later rejoined the reformed Exodus from 2002 to 2004. He again parted ways with the band during their South American tour later in 2004; according to Holt, Zetro bailed from the tour just as the tour was about to start. This caused the split to be fairly hostile; in retrospect, Zetro has admitted the blame for the circumstances, saying that it was his discontent at the time that caused the problems.
Thrash Against Cancer
Souza hosted the Thrash Against Cancer benefit in 2005 at The Pound in San Francisco, and appears onstage with Testament every time they play the Bay Area.
Dublin Death Patrol
In 2006, he and Chuck Billy formed Dublin Death Patrol, an American thrash metal band. Alongside Willy Lange (Lȧȧz Rockit). In 2007, they went on a world tour and did Fields of Rock in the Netherlands alongside Heaven & Hell, Korn, Iron Maiden, Ozzy Osbourne, Machine Head, Black Label Society, Hatebreed, Megadeth, Papa Roach, Slayer, Motörhead, Mastodon, Velvet Revolver, Dream Theater, Suicidal Tendencies, Amon Amarth, DragonForce, Ill Niño, and DevilDriver.
In August 2008, Souza became the lead singer for the metal band TENET along with members Byron Stroud (Strapping Young Lad, Fear Factory, Zimmer's Hole), Jed Simon (Strapping Young Lad, Zimmer's Hole), Glen Alvelais (Testament, Forbidden) and Gene Hoglan (Death, Unearth, Dethklok, Strapping Young Lad, Dark Angel, Testament, Zimmer's Hole, Old Man's Child). The album was eventually released in July 2009.
Souza, along with fellow members Kosta Varvatakis, Justin Sakagawa and his two sons Nicholas Souza and Cody Souza, started a thrash metal project called Hatriot. Hatriot released their debut album, Heroes of Origin, on January 25, 2013. The group's second album, Dawn of the New Centurion, was released on February 21, 2014 via Massacre Records. Steve eventually left Hatriot due to Exodus touring duties in 2015 and was succeeded by his son Cody.
Return to Exodus
On June 8, 2014, it was announced that Souza had rejoined Exodus. He appeared on Exodus' tenth studio album Blood In, Blood Out, which is his first album with the band since 2004's Tempo of the Damned.