Born on September 6, 1876 at Cluny, in Perthshire, Scotland. He was the son of the Rev. Robert Macleod.
In 1899 he was appointed Demonstrator of Physiology at the London Hospital Medical School and in 1902 he was appointed Lecturer in Biochemistry at the school. In 1903 he was appointed Professor of Physiology at the Western University at Cleveland, Ohio.
Macleod's main work was on carbohydrate metabolism and his efforts with Frederick Banting and Charles Best in the discovery of insulin. For this Banting and Macleod were jointly awarded the Nobel Prize for Medicine[?] in 1923.
He wrote eleven books, including Recent Advances in Physiology (1905); Diabetes: its Pathological Physiology (1925); and Carbohydrate Metabolism and Insulin (1926)
He died on March 16, 1935.
There is a persistent rumour that he did not participate in the discovery of insulin. It is possible that he lent his lab facilities to Banting and Best and was away on a fishing trip when the discovery was actually made.