Born in Rome, New York, Brooks attended college at the University of Rochester and subsequently studied homeopathic medicine in New York City. He dropped out after two years however, and returned to Rochester where he married his first wife, Anne Shepard, in 1909. Brooks found employment with an advertising agency in Utica, then "retired" in 1911, evidently because he came into a considerable inheritance. His retirement was not permanent: in 1917, he went to work for the American Red Cross and later did editorial work for several magazines, including The New Yorker. In 1940, Brooks turned to his own writing for his full time occupation. Walter married his second wife, Dorothy Collins, following the death of Anne in 1952.
The first works Brooks published were poems and short stories. His short story "Ed Takes the Pledge" about a talking horse was the basis for the 1960s television comedy series Mister Ed (credit for creating the characters is given in each episode to "Walter Brooks"). His most enduring works, however, are the 26 books he wrote about Freddy the Pig and his friends.