His mother, Kathy Pepino Guarini, is Italian American, and was a journalist for WTVM TV in Columbus, and later for CNN.

The group released an independent album in 1999 with proceeds benefiting a music scholarship fund at Guarini's alma mater, Central Bucks High School East in Buckingham, Pennsylvania.

His high school and college theatre credits include the lead roles in The Mystery of Edwin Drood, The Little Prince, The Pirates of Penzance, The Taming of the Shrew, and Once on This Island.

Guarini wrote about his decision between the two offers in his exclusive 2008 blog for the television site, Fancast, stating, "I had attended master classes… I learned Zulu and Sotho phrases, as well as the music from the show.

[6] Guarini had a role in the 2004 pre-Broadway New York Stage and Film Powerhouse Theater workshop/presentation of the musical Good Vibrations at Vassar College.

His performance (which included singing, dancing, acting, and playing guitar) landed Guarini an offer to continue the role in the original Broadway production, which he eventually declined.

[7] Guarini co-produced his second album Stranger Things Have Happened, independently released in December 2005 through his own production company, Justice Entertainment, Ltd..

Gayuna Cealo, and a song for the documentary series Project Everlasting, in August 2006 and featured on the Today Show.

In May 2008, Guarini released an acoustic EP titled Revolve comprising three songs ("Sweet Time", "Rock Bottom", and "When She Cries") co-written and co-produced with Shaun Ingram.

While stepping out to support the opening of The American Idol Experience at Disney's Hollywood Studios[8] in February 2009, Guarini confirmed his impending marriage to his fiancée, Reina Capodici.

[11][12][13] Guarini played Will, one of the three male leads in Green Day's rock musical American Idiot performing March 1, 2011, through the show's closing night, April 24, 2011.

[18][19] Other stage work includes lead roles in regional productions of Rent, Chicago, Company and Joseph and the Amazing Technicolor Dreamcoat.

[20][21] In It's a Wonderful Life, A Live Radio Play Guarini voiced George Bailey in a 2015 on-stage comedic production.