As he grew up, Thurber's writing skills became evident. He was chosen to write the "Class Prophecy" for the eighth graders of 1909, an essay that included the name of every one of his classmates in a tale set in the future. His writing both impressed his teachers and won admiration from his peers.

Thurber wrote many books and articles for The New Yorker magazine throughout his lifetime. He even wrote a few screenplays, although most were flops at the box office. Perhaps Thurber's best and most famous work was a 2500-word story called "The Secret Life of Walter Mitty." In the words of Burton Bernstein, who wrote a biography of the humorist, Thurber attained the "supreme distillation" of all his characters and themes: "the emasculated, daydreaming little man, a would-be Conradian figure hiding in a three-button suit; the emasculating, practical wife, a virago hiding inside a shrew; the love-fear of modern machinery; the attraction of fantasy as a release from reality; and, as always, the fascination of words. Here it all was, put together clearly, brilliantly, definitively."

Thurber said that Mitty was based on "every other man I have known," although close friends can see a great amount of James's father and brother William in the character. Walter Mitty has become a man whom countless people can, and do, identify. A footnote in Neil A. Grauer's biography of Thurber reads:

Actor D. B. Sweeney told CNN News n 1992 that he enjoyed "the Walter Mitty experience" of skating with professional hockey players in the film The Cutting Edge. On the March 22, 1992, broadcast of ABC News, Forrest Sawyer introduced a story about would-be pleasure boat builders by saying, "All of us have our Walter Mitty dream worlds." And in an interview for the 1992 documentary series "The Class of the Twentieth Century," former Washington Post editor Benjamin Bradlee, who oversaw the paper's Pulitzer Prize-winning investigation of the Watergate scandal, said he still dreamed "in my Walter Mitty way" of interviewing Richard Nixon. Nixon himself, wrote Henry Kissinger, "lived out a Walter Mitty dream of toughness that did not come naturally."