Thornton was born at Woodhurst, Pease Pottage, Sussex, son of Edward Thornton (1856-1904), a diplomat who worked in Buenos Aires, Russia and the United States, and Emma Jessie, daughter of Philip Rawson, DL, of Woodhurst, Sussex.
His grandfather was the diplomat Edward Thornton, 2nd Count of Cacilhas; on his grandfather's death in 1906, Thornton succeeded to the title as 3rd Count of Cacilhas (also "Cassilhas") due to his father's death two years earlier.
[1][2] Thornton served during the First World War with the Royal Fusiliers, holding the rank of temporary lieutenant in August 1916.
He remained in the Royal Air Force into the 1940s, serving during the Second World War, during which he retired in March 1945, retaining the rank of group captain.
[8] In 1922, Thornton married Marjorie Gabriel Pike, daughter of W. R. Pike;[9][10] they had three sons - Edward Peter, Christopher, and David - and a daughter, (Marjorie) Jean, who married Royal Navy lieutenant Ralph Carnac Baker Stallard-Penoyre (1913-1943), of a landed gentry family of Herefordshire.