Born in Atlanta, Georgia, Guarini started his musical career at the age of four, when he began to sing with the Atlanta Boys Choir. After graduating from Central Bucks East High School (Doylestown, Pennsylvania) Guarini attended college in Philadelphia, then moved to New York City where he studied at the School for Film and Television. He worked briefly on Broadway before auditioning for ''American Idol in April of 2002.
Guarini placed second in the American Idol competition, ultimately losing to Kelly Clarkson. Guarini did secure a record deal with RCA, and released his first album, Justin Guarini, in June of 2003.
The young singer's career seemed to come crashing down in 2003, when his first feature film (From Justin to Kelly) turned out to be a box-office flop. His debut album fared little better; six months after release, it had sold only about 130,000, whereas fellow American Idol singers Kelly Clarkson and Clay Aiken had sold over a million albums each in a fraction of the time. The album was panned even by American Idol judges Simon Cowell and Randy Jackson; Jackson commented: "Everyone who had a hand in this catastrophe, y'all really should rethink your jobs." Around December of 2003, Guarini was quietly dropped by RCA Records.