The Tony award-winning American actor is best known for playing Dr James Wilson in the Fox television series House and Neil Perry in the 1989 movie Dead Poets Society.
Having frequently performed on and off Broadway since 1986 in production including The Invention of Love (for which he won a Tony Award) and Long Day's Journey Into Night (for which he was Tony nominated), To Kill a Mockingbird marks Leonard's return to the London stage for the first time in 22 years.
Further casting includes Richie Campbell (Tom Robinson), Simon Gregor (Bob Ewell), Hattie Ladbury (Ms Maude Atkinson), Rona Morison (Mayella Ewell), Joe Speare (Reverend Sykes) and Daniel Tuite (Boo Radley). Eleanor Worthington-Cox, one of the original Matildas, will share the role of Scout.
Harper Lee's story of a court case in the US marred by racism won a Pulitzer Prize in 1960 and is widely regarded as a classic of American literature. Artistic director Timothy Sheader will direct Christopher Sergel's adaptation, with designs by Jon Bausor. To Kill a Mockingbird opens the 2013 season on 16 May (running until 15 June 2013).