His father was Simon Flexner, a sixth-grade dropout who became a self-taught microbiologist, pathologist, director of the Rockefeller Institute for Medical Research in New York City and discoverer of a cure for spinal meningitis.

His mother was Helen Thomas [Flexner], a professor of English at Bryn Mawr whose sister was president of the college.

[3][5] In 1929, Flexner graduated cum laude from Harvard University, and found work as a reporter for the New York Herald Tribune.

He and his father, Simon Flexner, M.D., co-wrote William Henry Welch and the Heroic Age of American Medicine (1941).

(His uncle, Abraham Flexner, was the educator whose 1910 report led to the reform of United States medical schools.)