Leonard was born in Toronto to parents Harry and Florence. He grew up in a working class street running north-south between Kingston Road and Gerrard Streets, in an area that is now known as the Upper Beach. By all accounts, Leonard was a happy, active child who loved playing football and other sports.

At age 11, Leonard received a diagnosis of diabetes mellitus. His prognosis was very poor. At the time, the only method of managing the symptoms of diabetes was to keep blood sugar down by limiting food intake. Leonard starved himself to stay alive. This worked for a few years, but by age 14, the 5’11” boy weighed only 65 pounds and was in very poor health. Harry and Florence brought Leonard to Toronto General Hospital in early December 1921 on the verge of entering a diabetic coma. Desperate to save their son, Harry and Florence agreed to allow Leonard to participate in an experiment to test the treatment developed by Frederick Banting, Charles Best, James Collip and J.J.R. Macleod.

The first injection in mid-January did not work, but after some refinements to the process of isolating insulin, Leonard was injected again in late January. Almost immediately, his blood sugar dropped and his symptoms disappeared. He went home in May of 1922. Further refinements of the treatment continued, and Leonard was put on insulin permanently in October of 1922.

Little else is known about Thompson, except that he eventually died of complications from pneumonia 13 years later.

Although not widely known, Leonard Thompson was the first patient to have his life prolonged by insulin. Elizabeth Hughes, whose father was the Secretary of State in the United States of America, is arguably the most famous of the early insulin patients. And yet Leonard, coming from a humble home, whose parents would try anything to save him, represents one of the other legacies of insulin. That it be available for all who need it.
— Written by Kirsten Vanstone