Kirby has been performing since his teens after he was accepted at The National Theatre School of Canada. He graduated in May 2000 and after two auditions he began working on two separate projects: the CBS/Alliance mini-series Haven and director Léa Pool's feature Lost and Delirious. Soon after, Kirby performed the role of Morgan in the Factory Theatre's production of Geometry in Venice in Toronto, a performance that garnered him a Best Actor nomination at the Dora Mavor Moore Awards. This was quickly followed by the role of Patroclus in Theatre for a New Audience's production of Troilus and Cressida directed by Peter Hall in New York City. In 2006, he performed in the Women's Project Theater production of Jump/Cut. Other theatre credits include Judith Thompson's premiere of Habitat at Canadian Stage followed by Daniel Brooks premiere of The Good Life at the Tarragon Theatre. His latest theatre venture was in NYC where he performed the lead role in Defender of the Faith. Kirby's first feature film was the role of Jim in . Other film credits include lead roles in Peter Wellington's feature Luck and Mambo Italiano directed by Émile Gaudreault. Mambo Italiano received a gala presentation at the 2003 Toronto International Film Festival to a standing ovation and earned Kirby a Canadian Comedy Award Nomination. Kirby then performed in a part written for him in the feature film Shattered Glass produced by Cruise/Wagner. In 2007, he played the lead role of Ray Dokes opposite Rachael Leigh Cook and Keith Carradine in the Canadian feature All Hat. A 2009 film role had him opposite Lindsay Lohan in a feature titled Labor Pains. In television, one of Kirby's favourite roles was the TMN/Showcase mini-series Slings and Arrows, featuring Canadian actors and directed by his friend Peter Wellington. Sex Traffic, a mini-series for Channel 4 and CBC that aired in the fall of 2004, had him working with British director David Yates, for which he received a Gemini nomination. That same year he received a second Gemini nomination for his guest starring role in the dramatic seriesThe Eleventh Hour. In the fall of 2005, Kirby appeared as a series regular in HBO's Tell Me You Love Me, directed by Patricia Rozema. The first season aired in HBOin September 2007. He starred as Jimmy Burns in the 2009–2010 Canwest Global television series Cra$h & Burn. In 2017, Kirby joined the cast of the Golden Globe winner The Marvelous Mrs. Maisel, an original series from Amazon, as the comedian Lenny Bruce, for which he received a Primetime Emmy Award for Outstanding Guest Actor in a Comedy Series. From December 2019 through January 2020, Kirby starred in the Off-Broadway play "Judgment Day" at New York's Park Avenue Armory.
