Born in London in August 1927, in the mid-1960s de Keyser worked twice with the writer, actor and director Jane Arden.
They acted together again in another Jane Arden script in the film Separation (Jack Bond 1968) which was set in London and featured music by Procol Harum, Matthew Fisher and Stanley Myers.
He starred in the BBC Radio 4 comedy The Attractive Young Rabbi with Tracy-Ann Oberman.
He also made an appearance in the British TV series The Professionals, in the episode entitled "Servant of Two Masters".
De Keyser was the narrator for British Pathé Pictorial in the 1960s, and also did voiceover work on television advertisements in the United Kingdom, as well as served as the announcer on the first series of comedy panel game Would I Lie to You?, before being replaced for the second series.