In 1971, MacLeod developed and became the director of the United States' first federal Health Maintenance Organization (HMO) program.

He was recruited by Elliot Richardson, former secretary of the U.S. Department of Health, Education and Welfare.

In 1972–1973, MacLeod carried out a Ford Foundation study of three European health care systems in Britain, Germany, and Denmark, while residing in Geneva, Switzerland, with his wife and two sons for six months, from October 1 to March 31.

He criticized Pennsylvania's preparedness, in the event of a nuclear accident, at the time for not having potassium iodide in stock, which protects the thyroid gland in the event of radiation exposure, as well as for not having any physicians on Pennsylvania's equivalent of the nuclear regulatory commission.

When McLeod announced nine months after the accident that child mortality in a ten-mile radius around the plant had doubled, he was fired by the governor.