Educated and trained as an actor in Canada at the Ryerson University School of Theatre in Toronto, McCormack has been appearing in television productions since his debut in 1986.
He played Col. Francis Clay Mosby in the 1-hour Western television miniseries Lonesome Dove: The Series (1994), and Lonesome Dove: The Outlaw Years (1995). In the show, which ran for two seasons, McCormack puts on a convincing Southern accent. The show was filmed at the CL Western Town Studio in Alberta.
His role as gay lawyer Will Truman in the NBC sitcom Will & Grace from 1998 until 2006 has brought him widespread recognition. Nominated for Emmy Awards on three occasions for this role, he won the award for Outstanding Lead Actor in a Comedy in 2001.