From the 1910s until his death, Van Loon wrote many books, illustrating them himself. Most widely known among these is The Story of Mankind, a history of the world especially for children, which won the first Newbery Medal in 1922. The book was later updated by Van Loon and has continued to be updated, first by his son and later by other historians.
However, he also wrote many other very popular books aimed at young adults. As a writer he was known for emphasizing crucial historical events and giving a complete picture of individual characters, as well as the role of the arts in history. He also had an informal and thought-provoking style which, particularly in The Story of Mankind, included personal anecdotes. As an illustrator of his own books, he was known for his lively black-and-white drawings and his chronological diagrams.