Joseph Francis III Mazzello was born on September 21, 1983 in Rhinebeck, New York, and was further raised in Hyde Park, New York, the son of Virginia and Joseph Mazzello Jr., who owned a performing school. He has one older sister, Mary, and a younger brother, John, who have both appeared in films. His father is of three-fourths Italian and one-fourth Jewish ancestry, while his mother is of Irish, Polish, and English descent.
Mazzello went to Catholic school Our Lady of Lourdes. He is an alumnus of the University of Southern California, entering the USC School of Cinematic Arts in 2001 following a recommendation letter of director Steven Spielberg. His favourite actor is his idol James Stewart and, favourite movie is It’s a Wonderful Life (1946).
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 my graduation present from Spielberg. His first film appearance was a small role Presumed Innocent (1990), starring Harrison Ford. He followed up that success with a supporting role as an abused child in the drama Radio Flyer (1992).
His most notable roles were as Douglas Gresham in Shadowlands and Tim Murphy in Jurassic Park, both in 1993, Dexter in the 1995 film The Cure, and Sledgehammer Eugene Sledge in the HBO miniseries The Pacific. In 2010, Mazzello played Dustin Moskovitz, one of the co-founders of Facebook in the David Fincher directed film The Social Network.
Mazzello made his directorial debut with the short film Matters of Life and Death (2007) in which he also stars; the cast includes David Strathairn (with whom he has worked in The River Wild (1994), Simon Birch (1998), and The Sensation of Sight (2006), Daniel Gillies, and Rachael Leigh Cook. Apart from movies, he has also worked in number of television series including: Province (2002), Without a Trace (2004), The Pacific (2010), Justified (2013), Person of Interest (2014) and many more.
In his entire career he got nominated for the Young Artist Award twice in 1992 and 1995 for Radio Flyer and, The Cure. He also won the Hollywood Film Festival Award for Best Cast for the movie The Social Network in 2010.