He is noted for his role in the discovery and isolation of insulin during his tenure as a lecturer at the University of Toronto, for which he and Frederick Banting received the 1923 Nobel prize in Physiology or Medicine.

He was awarded his medical degree with honours in 1898 and then spent a year studying biochemistry at the University of Leipzig, Germany, on a travelling scholarship.

He researched various topics in physiology and biochemistry, among which were the chemism of Mycobacterium tuberculosis, electroshocks, creatinine metabolism and blood circulation in the brain.

[1][6][11] At the end of 1920, Macleod was approached by Frederick Banting, a young Canadian physician who had the idea of curing diabetes using an extract from a pancreas whose functioning had been disrupted.

Even though Banting had virtually no experience of physiology, he managed to convince Macleod to lend him laboratory space during a holiday in Scotland that summer.

From Banting's viewpoint, "this was a brazen coup by Macleod to rob him of the credit for having discovered insulin – and to rub salt into the wound, it had been done in front of the most eminent doctors in the field".

This convinced Macleod to divert the whole laboratory to insulin research and to bring in the biochemist James Collip to help with purifying the extract.

[14] Although all the team members were listed as co-authors of their publications, Banting still felt overlooked, because Macleod took over the coordination of clinical trials and the acquisition of larger amounts of extract.

Macleod's presentation at a meeting of the Association of American Physicians in Washington, D.C., on 3 May 1922 received a standing ovation, but Banting and Best refused to participate in protest.

At that time, demonstrations of the method's efficiency drew huge public interest, because the effect on patients, especially children, who until then were bound to die, seemed almost miraculous.

[3] The pharmaceutical company Eli Lilly & Co. took over mass production, but without an exclusive license, as the patent was transferred to the Medical Research Council to prevent exploitation.

Working at the Marine Biological Station in St. Andrews, New Brunswick, he made extracts from each of those parts separately and proved that insulin is derived from the insular and not the acinar tissue of the pancreas.

Banting eventually started to claim that he deserved all the credit and that Macleod had only hindered him the whole time and had made no contribution other than to leave the keys to the laboratory when he went on vacation.

[3] MacLeod wrote a report on the discovery in 1922 to explain his side of the story, but otherwise refrained from active involvement in controversy about credit.

His last major contribution was a proof that the central nervous system does have an important role in maintaining carbohydrate metabolism balance, as was his original hypothesis.

The 1976 ITV television drama Comets Among the Stars, with Ralph Richardson in the Macleod role, portrayed him as dark and repulsive.

In the autumn of 1923, Banting and Macleod received the Nobel Prize in Physiology or Medicine, even though the long-term importance of the discovery was not yet apparent.

They were nominated by the Danish physiologist and Nobel laureate August Krogh, who had a diabetic wife and had visited Macleod's laboratory and taken the method back to Denmark.

However, "among experienced scientists there was more support for the view that Banting and Best’s somewhat fumbling researches would not have reached the goal without the contributions of both Macleod and Collip".

[3] A second controversial aspect of the award was the fact that eight months before Banting's and Best's paper, the Romanian physiologist Nicolae Paulescu had reported the discovery of a pancreas extract that he dubbed pancrein, which lowered blood glucose concentration.