His other theater credits include Candida, Long Day's Journey Into Night, Breaking the Code, The Speed of Darkness, Philadelphia, Here I Come!, Arcadia, The Music Man, Born Yesterday, Fifth of July, and To Kill a Mockingbird.

[5] Leonard is a three-time Tony Award nominee, and won Best Featured Actor in a Play in 2001 for his role as A. E. Housman in Tom Stoppard's The Invention of Love.

He was nominated for a Tony in 2003 for his portrayal of Edmund Tyrone in a well-received revival of Eugene O'Neill's Long Day's Journey Into Night, co-starring Philip Seymour Hoffman, Brian Dennehy, and Vanessa Redgrave.

[6] Leonard and Ethan Hawke, who have been friends since childhood, co-founded New York's Malaparte theater company alongside James Waterston, Steve Zahn, and Frank Whaley in 1991.

Entertainment Weekly said that Leonard "does a first-rate job of juggling Danny's mixture of despair, neediness, and mordant jokiness.

"[12] From 2004 to 2012, Leonard had his most famous onscreen role as Dr. James Wilson in the Fox medical drama series House.

"[13] He believed that his House audition was not particularly good, but that his friendship with the show's executive producer Bryan Singer helped win him the role.