[1][2] Leaf studied at the University of Maryland where he had played lacrosse and served as class treasurer, graduating in 1927.

[5] Leaf once commented, "Early on in my writing career I realized that if one found some truths worth telling they should be told to the young in terms that were understandable to them."

The story, which follows a gentle bull in rural Spain who prefers smelling flowers to bullfighting, sparked considerable controversy because Ferdinand was regarded by some as a pacifist symbol.

Leaf and Lawson's second collaboration, Wee Gillis, about a boy living in Scotland halfway between his father's family in the Highlands and his mother's in the Lowlands, was cited as a 1939 Caldecott Honor Book.

In the 1930s and 1940s Leaf wrote a regular feature for The American Magazine, titled "Streamlined Samples of the World's Best Stories," offering one-page, jocular, off-the-cuff condensations of Ivanhoe, Robinson Crusoe, Romeo and Juliet and others.

During World War II, Leaf worked for the Army Department and after the war, he volunteered his skills to the State Department, insisting he was "anxious to work with the [Office of Public Affairs] (without compensation and in an unofficial capacity)...on international policy matters".

Leaf died from cancer at age 71 in his home in suburban Garrett Park, Maryland, on December 21, 1976.