Robert (Henry Stewart) Thompson was born in Croydon on 2 February 1912, the youngest of three children, but the only son. There seems never to have been any doubt as to the professional destiny of Robert for his father, Joseph Henry, at the time of Robert's birth was a successful general practitioner in Croydon and a member of the honorary staff of the Croydon General Hospital where he served as Senior Physician and Chairman of the Medical Committee until 1939. Robert's father was educated in Londonderry and read medicine at Queen's University, Belfast, but left Ireland after he qualified. His mother, Mary Ellen Rutherford, was the eldest daughter of Dr Robert Rutherford. After Robert's birth his mother became a victim of multiple sclerosis but survived until 1951, after which Robert's father remarried, living to the age of 84. The home life in Croydon was that of a well–to–do doctor, for they had a domestic staff, which included a chauffeur for the rather splendid two–seater Rover, a cook and a gardener as well as parlour maids.