John McLeod was born on September 6, 1876 in Scotland. He took his medical degree with honours and was later appointed Professor of Physiology at the University of Toronto.

Before his collaboration with Frederick Banting and Charles Best, Macleod’s research focused on carbohydrate metabolism. In 1923, Banting and Professor Macleod were jointly awarded the Nobel Prize for Physiology or Medicine. Macleod shared it with his colleague James Collip. Macleod enjoyed a successful career as a teacher and director of research and was honoured by many scientific groups and societies. He married Mary McWalter. He died in 1935 at the age of 58.