Zack was interested in punk rock- including bands such as the Sex Pistols and the Clash- from a young age. In seventh grade, Zack played guitar in a punk band called Juvenile Expression, along with his friend Tim Commerford, who played bass. As a teenager, Zack grew to like hardcore punk bands such as Minor Threat and Fugazi, and joined Hardstance as a guitarist. In 1988, Hardstance changed it's name to Inside Out, and Zack became vocalist. Inside Out broke up in 1991. In August of that year, while performing in a club, Zack was approached by Los Angeles local musicians Tom Morello, a guitarist, and Brad Wilk, a drummer and the three agreed to form a band called Rage Against the Machine, which was the name of a song Zack had written when he was vocalist. De la Rocha asked former Juvenile Expression bandmate Tim Commerford to play bass, and he agreed.