Discovered by Rosie O'Donnell, he was one of the prominent sketch comedy stars on the groundbreaking Logo TV series The Big Gay Sketch Show (2007–2010) with fellow comedians Kate McKinnon, Erica Ash, Colman Domingo, Jonny McGovern, Julie Goldman, Paolo Andino, Dion Flynn, and Michael Serrato.

He also appeared on The Wedding Bells, 'Til Death, The Neighbors, The Comeback, Marry Me, Finding Carter, Jessie, Sofia the First, Superior Donuts, 2 Broke Girls.

He played Connor on ABC's Dr. Ken, Quincy on Netflix's EastSiders (Daytime Emmy nomination), and Sully Patterson on Jim Carrey's Showtime drama I'm Dying Up Here.

[13] Notable alumni include Tiffany Haddish, Kate McKinnon, Fortune Feimster, Justin Hires, Nicole Byer, Nasim Pedrad, John Milhiser.

[16][10] In 2015, Guarino co-produced and co-wrote a CBS sketch comedy pilot called The Night Crew with Michael Serrato, Steven Borzachillo, Mark Hurtado, and Ryan Noggle David Burtka, and Neil Patrick Harris starring himself, Tiffany Haddish, Fortune Feimster, Jimmy Ray Bennett, and Nico Santos.