St. Aubyn taught history at Eton College for 40 years. He was a nephew of Vita Sackville-West and personal friend of John Beetjeman and John Le Carré.
He was considered an eccentric and was reserved, bordering on shy, and had an understanding of human nature which made him an accomplished biographer.
In his will be bequested money for the establisment of the RLS Giles St. Aubyn Award for Non-Fiction, which is awarded to writers working on their first book to help fund their research.