After graduating with his bachelor's degree, Martin taught journalism, drama, and English at high schools in Newton and St. John, Kansas. During World War II, he served in the Army Air Force as a newspaper editor and wrote his first book, "The Little Squeegy Bug", published in 1945, as William Ivan Martin, with illustrations by his brother Bernard Martin. Eleanor Roosevelt praised the book in her syndicated newspaper column, "My Day," and it eventually sold 1 million copies. He wrote 10 more books with his brother and by the time of his death had published more than 300 children's books, always working with a carefully chosen illustrator. He liked to collaborate and to make many revisions until the words sounded right. Martin then worked as principal of Crow Island Elementary School in Winnetka, Illinois, and later moved to New York City and joined the publishing company Holt, Rinehart and Winston, where he was editor in chief of the school division during the 1960s and developed innovative reading programs.