He published 46 children's books, which were often characterized by imaginative characters, rhyme, and frequent use of anapestic meter. His most celebrated books include the bestselling Green Eggs and Ham, The Cat in the Hat, One Fish Two Fish Red Fish Blue Fish, Horton Hatches the Egg, Horton Hears a Who!, and How the Grinch Stole Christmas!. Numerous adaptations of his work have been created, including 11 television specials, four feature films, a Broadway musical and four television series. He won the Lewis Carroll Shelf Award in 1958 for Horton Hatches the Egg and again in 1961 for And to Think That I Saw It on Mulberry Street.
Geisel also worked as an illustrator for advertising campaigns, most notably for Flit and Standard Oil, and as a political cartoonist for PM, a New York City newspaper. During World War II, he worked in an animation department of the United States Army, where he wrote Design for Death, a film that later won the 1947 Academy Award for Documentary Feature.