Leonard was born in Houston, Texas, the son of Joann, an author, and Robert Leonard, a theater professor at Pennsylvania State University. He was raised in State College, Pennsylvania. Leonard obtained his GED and at age 16 travelled to Chiapas, Mexico, where he taught at an elementary school and volunteered at an anthropological institute. Leonard then spent time in Seattle and London, where he developed an interest in photography, before settling in New York City at age 19. He is Jewish on his mother's side.
In 1997, Leonard auditioned for and landed one of the three principal roles in the found footage horror film The Blair Witch Project. Although he reportedly only earned an initial salary of $500, the experience proved rewarding in other ways as the film combined acting and photography. The film premiered at the Sundance Film Festival. Leonard has since appeared in numerous films and television shows as an actor, including HBO's television film Live from Baghdad, the drama film Things Behind the Sun directed by Allison Anders, the comedy film The Shaggy Dog with Tim Allen, the drama film Men of Honor with Robert De Niro and Cuba Gooding, Jr., and the psychological thriller film Bitter Feast with Larry Fessenden.
In 2009, he starred in the Independent Spirit Award-winning film Humpday, for which he won Best Actor at the Gijón International Film Festival. Leonard has also appeared on the acclaimed HBO series Hung and the Showtime series United States of Tara. Leonard directed the short film The Youth in Us, which was shown at the 2005 Sundance Film Festival. Art documentary Beautiful Losers followed, winning the Grand Jury Prize at CineVegas in 2008. He has also directed music videos for many popular acts, such as Morcheeba, Harper Simon, and Fitz and the Tantrums, as well as teaching directing and acting at The New York Film Academy, UC Irvine, and Academia Internacional De Cinema in São Paulo, Brazil. In 2011, Leonard starred in Vera Farmiga's directorial debut drama film Higher Ground, playing a religious man at odds with his wife (Farmiga). He also co-wrote, directed and starred in The Lie, a devilish morality tale adapted from the story by acclaimed author T. C. Boyle. The project co-starred Mark Webber, Jess Weixler, and Jane Adams, and was well received by audiences and critics.