[1] Rather than take up his place at the University of Cambridge, with the outbreak of World War I, Whitby joined the Royal Fusiliers, British Army, as a private.

[1][4] He was haemorrhaging from the femoral artery but an American doctor managed to stop the bleed and Whitby survived the trauma.

[1][2] From 1928 to 1929, he was part of a team of medical staff who cared for the ailing King George V.[1][2][12] For the next ten years, he practised as a clinical pathologist and began his research into haematology.

[15] On 2 September 1939, he was called up for active service and transferred from the TA Reserve of Officers (RAMC) to the Territorial Army (Commands and Staff).

[16] He was promoted to brigadier on 1 March 1942, when he was commanding officer of the Army Blood Transfusion Service at Southmead Hospital in Bristol.

[21] In 1929, Whitby was appointed Commander of the Royal Victorian Order (CVO) in recognition of his role in the treatment of King George V.[1][22] In the 1945 New Year Honours, when he was commanding officer of the Army Blood Transfusion Service at Southmead Hospital in Bristol it was announced he was to be made a Knight Bachelor 'for services in the development of the sulphonamide group of drugs'.

[24] In 1938, he was awarded the John Hunter triennial medal and prize by the Royal College of Surgeons for his work in the development of the clinical use of sulphapyridine.

[3] In July 1945,[14] he was awarded the Gold Medal of the Royal Society of Medicine in recognition of his work on wound shock and blood transfusions.

The President of the RSM, Sir Gordon Gordon-Taylor, jokingly described Whitby as 'the greatest vampire the world has known'.