Abstract. John James Rickard Macleod, while sometimes remembered as a co-discoverer of insulin, was moreover one of the world's most accomplished academic physiologists in the early 1900s. A medical graduate in Aberdeen, Scotland, he pursued a career in physiology, travelling to Leipzig and London. Precocious progress in research, teaching, and writing saw him being appointed a physiology professor in Cleveland, Ohio, at the age of 27. He gained an international reputation in carbohydrate metabolism, publishing a monograph on diabetes in 1913. A move to Toronto in 1918 soon saw him leading the team that produced the world's first clinically useful, life-saving insulin. Despite sharing a Nobel Prize, his involvement in the insulin story was mired in unjustified and persistent controversy. Returning to Aberdeen in 1928 as the head of physiology, his wide-ranging success continued until his death in 1935. This article details his life, work, and many achievements both before, and after, the Toronto insulin years.