Arnatt trained for the stage at the Royal Academy of Dramatic Art.

In the 1962 film Dr Crippen, starring Donald Pleasence (who also had a recurring role in "The Adventures of Robin Hood" as Prince John), Arnatt played Chief Inspector Walter Dew.

[4] Never well known, he amassed numerous television credits in programmes such as Steptoe and Son, Keeping Up Appearances, Dangerfield, Lovejoy, The Professionals, House of Cards, Thriller and Z-Cars.

He had a recurring role in the first two series of the ITV legal drama, The Main Chance (1969–70) and in Doctor Who he was the second actor to play Time Lord Cardinal Borusa in the serial The Invasion of Time (1978).

Later, in the television film, Marple: The Moving Finger (1985), he played Reverend Guy Calthrop.