McKellar made his first screen appearance in 1989 in Bruce McDonald's film Roadkill, for which he also wrote the screenplay. McKellar's work on Roadkill earned him Genie Award nominations for best supporting actor and best Screenwriter, attracting the attention of many in Canada. Roadkill also won the Toronto-Citytv Award for best Canadian feature.
Since his entry into Canadian cinema, McKellar has also been involved in numerous projects. He appeared in Atom Egoyan's films The Adjuster (1991) and Exotica (1994), the latter of which earned him the Genie for best supporting actor. McKellar collaborated with François Girard, authoring the screenplays for his films Thirty Two Short Films About Glenn Gould (1992), and the Academy Award winning (Best Original Score) The Red Violin (1998), in which McKellar starred alongside Samuel L. Jackson. He also appeared alongside Jude Law and Jennifer Jason Leigh in David Cronenberg's 1999 film eXistenZ.
McKellar has emerged as a filmmaker in his own right; his directorial debut, Last Night (1998), garnered impressive critical acclaim, winning the Prix de la Jeunesse at the Cannes Film Festival and the Claude Jutra Award at the Genies. In 2001, he played the role of Oliver Tapscrew in the TV children's drama series I Was a Rat. His second film, Childstar, opened in 2004 at the Toronto International Film Festival to enthusiastic reviews.
McKellar also starred in the animated sitcom Odd Job Jack as the titular hero, Jack Ryder, which ran for four seasons between 2004 and 2007 on The Comedy Network.
In 2006, he appeared in Ken Finkleman's miniseries At The Hotel. In June 2006 he won the Tony Award for Best Book of a Musical and the Drama Desk Award for Outstanding Book of a Musical for The Drowsy Chaperone. He received a Gemini Award nomination for his role as socialist Politician Clarence Fines in Prairie Giant: The Tommy Douglas Story.
McKellar hosted the CBC Radio One series High Definition. He co-starred in and wrote the 2008 screen adaptation of José Saramago's 1995 novel Blindness.
McKellar has appeared in all three seasons of television's Slings & Arrows, as Darren Nichols, a theatre Director. The show is co-written by Bob Martin, who collaborated with McKellar on the musical The Drowsy Chaperone. Martin and McKellar also cocreated the Canadian television sitcom Michael, Tuesdays and Thursdays, scheduled to debut on CBC Television in fall 2011.
