John James Rickard Macleod was a Scottish physiologist and biochemist who is credited with the discovery of insulin, together with Canadian physicist Frederic Banting. The duo shared the Nobel Prize in Physiology or Medicine in 1923. Right from the early years, Macleod displayed a liking for physiology. He started his career as a demonstrator at the London Hospital Medical School and soon made his way to the top. A pioneer in scientific research, Macleod’s career kick-started much before the discovery of insulin. During the early years of his career, Macleod researched on caisson sickness. It was during this time that he developed an interest in carbohydrate metabolism in relation to diabetes. After 1905, he published 37 papers on carbohydrate metabolism and 12 papers on experimentally produced glycosuria. In 1920, Macleod first collaborated with Frederic Banting. Though the collaboration did not turn out fruitful personally between the two as their relation never blossomed, professionally, with the help of Macleod’s student Charles Herbert Best and biochemist James Collip they successfully discovered insulin. Over time, insulin became one of the most important medical discoveries of the 20th century. However, Macleod’s direct involvement in the discovery became a matter of contention. In his life, Macleod was a successful teacher, powerful orator and prolific writer. He penned close to 200 scientific papers and 11 books in his lifetime. Furthermore, he held various academic positions at prestigious institutes across the globe.