In 1986, Robert Sean Leonard made his Broadway debut in Brighton Beach Memoirs and his film debut in The Manhattan Project. His first starring film role was as a high-school vampire in the '80s teen comedy My Best Friend Is a Vampire (1988). But Leonardturned out to be perfect for the role of Neil Perry, the sensitive prep-school student whose acting aspirations are crushed by his wealthy father in the much-loved drama Dead Poets Society (1989). His next few films were period pieces: the Merchant-Ivory production Mr. & Mrs. Bridge (1990), Kenneth Branagh's Much Ado About Nothing (1993), and Martin Scorsese's The Age of Innocence (also 1993). Leonard also earned a Young Artist award for his performance in the WWII-era musical Swing Kids in 1993 and earned his first Tony nomination that same year for a revival of Candida. Though he often chose the stage over the screen, his theatrical training directed him toward roles in the talky feature films Married to It (1993), Safe Passage (1994), and The Last Days of Disco (1998). He also fared well in television adaptations of stage productions (The Boys Next Door [1996], In the Gloaming [1997]) and based-on-a-true-story docudramas (Killer: A Journal of Murder [1995], A Glimpse of Hell [2001]).

In 2001, Leonard reunited with Dead Poets Society co-star Ethan Hawketo appear in the independent drama Chelsea Walls, Hawke's directorial debut. He also co-starred with Hawke and Uma Thurmanin Richard Linklater's intensely talky drama Tape. After spending most of his career on the stage, Leonard finally earned a Tony award for his portrayal of A.E. Houseman in Tom Stoppard's The Invention of Love. Also on Broadway, he could be seen in A Long Day's Journey Into Night and The Violet Hour. Leonard briefly returned to television in 2003 for the starring role of a cotton farmer in the Hallmark Hall of Fame feature A Painted House. His 2004 projects would include the feature film The I Inside, based on the play Point of Death.

Currently, he plays Dr. James Wilson, head of the oncology department, on the FOX TV series House. Leonard maintains a close friendship with House co-star Hugh Laurie