 He was cast in his first Broadway show, "Seventeen", a musical adaptation of the Booth Tarkington novel that opened at the Broadhurst Theatre on June 21, 1951 and ran 182 performances.In 1960, Nelson was cast in an off-Broadway show entitled "The Fantasticks", which eventually became the world's longest-running musical with 17,162 performances.In 1962, he was hired to understudy Anthony Newley in "Stop the World - I Want to Get Off" when it transferred from the West End, eventually assuming the lead role when the star departed the show.