Guarini remained active in the performing arts during his school years. At Central Bucks High School East, he frequently performed with the a cappella group Midnight Voices. He also gained early entertainment experience through Cutting Edge Entertainment, where he performed at weddings, bar mitzvahs, and similar events. Later, he studied music and theatrical performance at the University of the Arts in Philadelphia and pursued acting training at the School for Film and Television in New York City.

His early exposure to diverse musical influences and performing arts helped shape his future career in music and acting. His first musical group was the Atlanta Boys Choir, and he shared classroom experiences with future stars such as singer Pink during junior high school.

Rise to Fame: American Idol and Entertainment Career

Guarini rose to national prominence in 2002 as the runner-up on the inaugural season of the television competition series American Idol. His soulful voice, charisma, and memorable performances made him a fan favorite on the show. His exposure on American Idol launched his career and opened doors in both the music and acting industries.

Following his success on American Idol, Guarini released a self-titled album and began performing in various capacities. He later transitioned into theater, auditioning for Broadway roles. In April 2010, he earned his first Broadway role as Carlos in the musical Women on the Verge of a Nervous Breakdown. He has also appeared in productions such as American Idiot and Wicked. Guarini performed at numerous charity and entertainment events, including a benefit performance at the Catalina Jazz Club in Hollywood alongside jazz musicians and singer Johnny Mathis.