He is best known for his roles as Tim Murphy in Jurassic Park, Roarke Hartman in The River Wild, Eugene Sledge in the HBO miniseries The Pacific, Dustin Moskovitz in The Social Network, and Queen bass player John Deacon in the biopic Bohemian Rhapsody.

[4] He is an alumnus of the University of Southern California, entering the USC School of Cinematic Arts in 2001 following a recommendation letter from director Steven Spielberg.

[5] Mazzello paid for school with his salary from a small appearance in The Lost World: Jurassic Park (1997), something the actor jokingly referred to as his graduation present from Spielberg.

He then went on to appear in Radio Flyer, Jersey Girl and the TV film Desperate Choices: To Save My Child in 1992.

Mazzello continued his film career with lead roles in Star Kid in 1997, and in Simon Birch with Ian Michael Smith in 1998.

In 2018, Mazzello portrayed John Deacon in the Queen biopic Bohemian Rhapsody, reuniting with his co-star from The Pacific, Rami Malek.