Katherine Applegate's many books include the Roscoe Riley Rules chapter book series, the picture book The Buffalo Storm, and the award-winning novel Home of the Brave. With her husband, Michael Grant, she wrote the hugely popular series Animorphs, which has sold more than 35 million copies worldwide.

Katherine was inspired to write The One and Only Ivan after reading about the true story of a captive gorilla known as Ivan, the Shopping Mall Gorilla. The real Ivan lived alone in a tiny cage for twenty-seven years at a shopping mall before being moved to Zoo Atlanta after a public outcry. He was a beloved celebrity at the zoo, which houses the nation's largest collection of western lowland gorillas. Ivan was well known for his paintings, which he "signed" with a thumbprint.

Katherine lives in California with her husband and two children.