Robert McCloskey was born on Sept. 14, 1914, in Hamilton, Ohio. He moved to Boston in 1932 after being awarded a scholarship to the Vesper George Art School. He then moved to New York to study at the National Academy of Design.

In 1940 he married Margaret (Peggy) Durand, daughter of children's author Ruth Sawyer. They had two daughters, Sarah and Jane, and settled in New York City, spending summers on Scott Island, Maine. That was the setting for his Caldecott Honor book, Blueberries for Sal, whose characters little Sal and her mother are reputed to be based on McCloskey's wife and eldest daughter Sarah.

McCloskey's wife Peggy died in 1991. Twelve years later, in 2003, McCloskey died at his home in Deer Isle, Maine. He was survived by his two daughters and by two grandchildren, Samantha and Seth.

Make Way for Ducklings was the 1942 Caldecott Medal winner. The book tells of a mallard family that comes to live in a pond in the Public Garden in the center of Boston, Massachusetts and how a friendly policeman stops traffic when the mother takes her eight ducklings across the street. This story has become an institution in Boston, and in 2003, it was named the official children's book of Massachusetts In 1987, sculptor Nancy Schön created a bronze version of Mrs. Mallard and the ducklings in the Public Garden, which are climbed on by thousands of children every year. The park is also the site of an annual Make Way for Ducklings Mother's Day parade, featuring hundreds of children dressed in the costumes of their favorite characters.

McCloskey won a second Caldecott Medal in 1958 for Time of Wonder. He also received Caldecott Honors for Blueberries for Sal in 1949, One Morning in Maine in 1953 and JourneyCake, Ho! in 1954. Marc Simont, another Caldecott Medal winner, said of McCloskey in a Horn Book Magazine article: "Bob McCloskey’s talent for devising mechanical contraptions is topped only by his ability to turn out books that carry off the Caldecott Medal."

A chapter from Homer Price was adapted into a short film, The Doughnuts (1963). The same chapter was adapted for an ABC Weekend Special called "Homer and the Wacky Doughnut Machine" (1977).

In 1964, film producer Morton Schindel and Weston Woods Studios (Norwalk, Connecticut) made the 18-minute Robert McCloskey, a documentary which is sometimes screened in art schools. The film shows McCloskey sitting in Boston Public Garden and intercuts pages from his sketchbook drawings for Make Way for Ducklings. The illustrator discusses experiences that have influenced his work and the relationship of craftsmanship to inspiration.

In McCloskey's hometown of Hamilton, Ohio, there is a statue depicting the title character and dog from his book Lentil (1940). In the book, the dog is unnamed, but after a competition among schoolchildren, the dog is now known as Harmony.

McCloskey was named a Living Legend by the Library of Congress in 2000.