He was born in Huddersfield, West Yorkshire, the son of Joseph Arnold Himsworth and Amy Eliza Barraclough. He was educated at the local Spring Grove School and King James's Grammar School, Almondbury. He married Charlotte Gray in 1932: they had two sons.
He studied medicine at the University of London and trained in University College Hospital (UCH). His early involvement in medical research (especially of diabetes and later of liver disease) would lead to an important 1936 paper in The Lancet, distinguishing the two main types of diabetes. He delivered the Goulstonian Lecture at the Royal College of Physicians in 1939 entitled Mechanism of diabetes mellitus
The Prime Minister of the day saw Sir Harold Himsworth's report about the 'committee considering the effects of ..... nuclear radiation' The Prime Minister's comment was "A pity, but we cannot help it". (dated 16 Nov 1955). From the office of AERE Directors Office.
His archives are being held by the Wellcome Library, London.
Himsworth recommended a high-carbohydrate diet to treat diabetes. Professor Edwin Gale has noted:
He demonstrated that injected insulin produced a greater hypoglycaemic response in individuals treated with the high carbohydrate diet, thus demonstrating that diet could influence insulin sensitivity. The high carbohydrate diet worked because it allowed the flow of glucose to the tissues to be maintained at a lower head of pressure by making people more sensitive to their own insulin.