During 1898 he received his medical degree from University of Aberdeen and went to work for a year at the University of Leipzig. During 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. During 1903 he was appointed Professor of Physiology at the Western Reserve University at Cleveland, Ohio. During 1918 he was elected Professor of Physiology at the University of Toronto, Canada.
Macleod's main work was on carbohydrate metabolism and his efforts with Frederick Banting and Charles Best in the discovery of insulin used to treat diabetes. For this Banting and Macleod were jointly awarded the Nobel Prize for Medicine in 1923. Macleod was awarded half of the Nobel Prize for the discovery of insulin, even though many people (including Banting) publicly insisted that Macleod's involvement was minimal and Best's work had been essential. However, it was MacCleod's research plan and his suggestion to inject intravenous degenerated pancreas into depancreatinized dog sugar that ultimately led to the successful isolation of insulin. There is currently a controversy regarding the role of Banting and Best in attempting to 'write out' Macleod and his colleague J.P. Collip from the history books. Macleod's receiving the Nobel Prize over Best was controversial at the time (see Nobel Prize controversies). He wrote eleven books, including Recent Advances in Physiology (1905); Diabetes: its Pathological Physiology (1925); and Carbohydrate Metabolism and Insulin. (1926)
Macleod shared his Nobel award money with J.P.Collip.