In 1919 he entered The Choate School (now Choate Rosemary Hall) in Wallingford, Connecticut. Originally intending to study chemical engineering, he became more interested in writing and worked as managing editor of the Choate Literary Magazine. He graduated in 1926 from Harvard, where he edited The Harvard Advocate, and where he studied with Charles Townsend Copeland.

In 1929, he published his first novel, Rome Haul, a work about the Erie Canal. The novel was adapted for the 1934 play The Farmer Takes a Wife and the 1935 film of the same name. He married Eleanor Stetson in 1930.

Drums Along the Mohawk was on the bestseller list for two years, second only to Margaret Mitchell's famous 1936 novel Gone with the Wind for part of that time. Bert Breen's Barn was a winner of the 1976 National Book Award.

Edmonds eventually published 34 books, many for children, as well as a number of magazine stories. He won the Newbery Medal in 1942, for The Matchlock Gun, and the National Book Award for Children's Literature in 1976, for Bert Breen's Barn.

When Eleanor died in 1956, Walter married Katherine Howe Baker Carr, who died in 1989. Walter Edmonds died in Concord, Massachusetts, in 1998.