Ellis attended Fitzwilliam College, Cambridge where he read history and appeared in around 20 plays.
Ellis' early roles include The Rivals at the Haymarket Theatre, The Real Inspector Hound and Widowers' Houses at the Royal Court. He was part of the innovative Actors' Company, founded in 1972 by Ian McKellen and Edward Petherbridge.
From 1975 to 1977, he portrayed Ross Poldark in the 1975 adaptation of Poldark.
Since then, Ellis has had appearances in many well known TV shows such as Heartbeat and Wallander.
Ellis has mainly appeared in theatre, and worked with the Royal Shakespeare Company for some time. He appeared there in a musical version of The Comedy of Errors playing Pinch; in King Lear playing Edmund; Troilus and Cressida playing Achilles; and in Much Ado About Nothing playing Don Pedro. His last West End theatre performance was with Zoë Wanamaker in Sylvia.
Ellis is the author of a memoir entitled Making Poldark about the 1975 series.