Leonard is acclaimed for his ear for dialogue and his gritty realism. Leonard counts Ernest Hemingway among his influences, though he said Hemingway didn't have a sense of humor. British novelist Martin Amis once told Leonard, "Your prose makes Raymond Chandler look clumsy."
Leonard's 10 rules for good writing are as succinct as his prose: 1) Never open a book with weather. 2) Avoid prologues. 3) Never use a verb other than "said" to carry dialogue. 4) Never use an adverb to modify the verb "said" … he admonished gravely. 5) Keep your exclamation points under control. You are allowed no more than two or three per 100,000 words of prose. 6) Never use the words "suddenly" or "all hell broke loose." 7) Use regional dialect, patois, sparingly. 8) Avoid detailed descriptions of characters. 9) Don't go into great detail describing places or things. 10.) Try to leave out the part that readers tend to skip.
His most important rule, Leonard says, sums up the 10. "If it sounds like writing, I rewrite it."
Leonard sold his first short story, Trail of the Apache, to Argosy magazine in 1951. It is set in Arizona in the 1880s, the days of U.S. Cavalry vs. the Apaches. It was published as the title piece in a collection of short stories in 2007. The title piece in another collection of short stories, Three-Ten to Yuma, was adapted as a film in 1957 and remade in 2007. Leonard's first novel was The Bounty Hunters (1953), also set in Arizona Territory.