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.

In 1968, Nelson accepted the lead in the controversial and groundbreaking off-Broadway production of The Boys in the Band, the first play to explore the milieu of gay life in New York City in a verbally frank manner.

[3] Also in 1970 Nelson returned to Broadway in the lead role in Lovely Ladies, Kind Gentlemen, a musical adaptation of The Teahouse of the August Moon.

Nelson appeared opposite Celeste Holm as Russell Paxton in the British premiere of the Kurt Weill / Ira Gershwin / Moss Hart musical Lady in the Dark at the Nottingham Playhouse on 9 December 1981.

[5] In 1985 he played sinister nuclear entrepreneur Jerry Grogan in Edge of Darkness, the seminal BBC thriller series written by Troy Kennedy Martin, and had roles in the Clive Barker horror films Hellraiser (1987) and Nightbreed (1990).