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Later years

After the war, Geisel and his wife moved to the La Jolla community of San Diego, California, where he returned to writing children's books. He published most of his books through Random House in North America and William Collins, Sons (later HarperCollins) internationally. He wrote many, including such favorites as If I Ran the Zoo (1950), Horton Hears a Who! (1955), If I Ran the Circus (1956), The Cat in the Hat (1957), How the Grinch Stole Christmas! (1957), and Green Eggs and Ham (1960). He received numerous awards throughout his career, but he won neither the Caldecott Medal nor the Newbery Medal. Three of his titles from this period were, however, chosen as Caldecott runners-up (now referred to as Caldecott Honor books): McElligot's Pool (1947), Bartholomew and the Oobleck (1949), and If I Ran the Zoo (1950). Dr. Seuss also wrote the musical and fantasy film The 5,000 Fingers of Dr. T., which was released in 1953. The movie was a critical and financial failure, and Geisel never attempted another feature film. During the 1950s, he also published a number of illustrated short stories, mostly in Redbook magazine. Some of these were later collected (in volumes such as The Sneetches and Other Stories) or reworked into independent books (If I Ran the Zoo). A number have never been reprinted since their original appearances.

In May 1954, Life published a report on illiteracy among school children which concluded that children were not learning to read because their books were boring. William Ellsworth Spaulding was the director of the education division at Houghton Mifflin (he later became its chairman), and he compiled a list of 348 words that he felt were important for first-graders to recognize. He asked Geisel to cut the list to 250 words and to write a book using only those words. Spaulding challenged Geisel to "bring back a book children can't put down". Nine months later, Geisel completed The Cat in the Hat, using 236 of the words given to him. It retained the drawing style, verse rhythms, and all the imaginative power of Geisel's earlier works but, because of its simplified vocabulary, it could be read by beginning readers. The Cat in the Hat and subsequent books written for young children achieved significant international success and they remain very popular today. For example, in 2009, Green Eggs and Ham sold 540,000 copies, The Cat in the Hat sold 452,000 copies, and One Fish, Two Fish, Red Fish, Blue Fish (1960) sold 409,000 copies—all outselling the majority of newly published children's books.

Geisel went on to write many other children's books, both in his new simplified-vocabulary manner (sold as Beginner Books) and in his older, more elaborate style.

In 1955, Dartmouth awarded Geisel an honorary doctorate of Humane Letters, with the citation:

Creator and fancier of fanciful beasts, your affinity for flying elephants and man-eating mosquitoes makes us rejoice you were not around to be Director of Admissions on Mr. Noah's ark. But our rejoicing in your career is far more positive: as author and artist you singlehandedly have stood as St. George between a generation of exhausted parents and the demon dragon of unexhausted children on a rainy day. There was an inimitable wriggle in your work long before you became a producer of motion pictures and animated cartoons and, as always with the best of humor, behind the fun there has been intelligence, kindness, and a feel for humankind. An Academy Award winner and holder of the Legion of Merit for war film work, you have stood these many years in the academic shadow of your learned friend Dr. Seuss; and because we are sure the time has come when the good doctor would want you to walk by his side as a full equal and because your College delights to acknowledge the distinction of a loyal son, Dartmouth confers on you her Doctorate of Humane Letters.

Geisel joked that he would now have to sign "Dr. Dr. Seuss". His wife was ill at the time, so he delayed accepting it until June 1956.

Geisel's wife Helen had a long struggle with illnesses. On October 23, 1967, Helen died by suicide. Eight months later, on June 21, 1968, Geisel married Audrey Dimond with whom he had reportedly been having an affair. Although he devoted most of his life to writing children's books, Geisel had no children of his own, saying of children: "You have 'em; I'll entertain 'em." Audrey added that Geisel "lived his whole life without children and he was very happy without children." Audrey oversaw Geisel's estate until her death on December 19, 2018, at the age of 97.

Geisel was awarded an honorary doctorate of Humane Letters (L.H.D.) from Whittier College in 1980. He also received the Laura Ingalls Wilder Medal from the professional children's librarians in 1980, recognizing his "substantial and lasting contributions to children's literature". At the time, it was awarded every five years. He won a special Pulitzer Prize in 1984 citing his "contribution over nearly half a century to the education and enjoyment of America's children and their parents".