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. From there, he went to another London import, Half a Sixpence, in 1965.