 On Broadway, Siberry has performed the likes of Nicholas Nickleby in "The Life and Adventures of Nicholas Nickleby", Gratiano in "The Merchant of Venice", Captain Georg von Trapp in "The Sound of Music" and Shakespeare in "The Frogs" (Lincoln Center).Other theater credits include Morrell in "Candida" and Astrov in "Uncle Vanya" at the McCarter Theatre in Princeton, New Jersey and Osbourne in "Journey's End", Oberon in "A Midsummer Night’s Dream" and Billy Flynn in "Chicago" at London's West end theatre.He portrayed King Arthur in the U.S. National Tour of "Monty Python's Spamalot" for two years before reprising it on Broadway in 2008.