An American author of fantasy novels, he is best known for works such as Silverlock and The Moon's Fire-Eating Daughter. He also wrote several works of historical fiction.
He wrote for both the New York World and the San Antonio Evening News.
He published nearly twenty books over the course of his career.
He married Charlotte Shanahan in 1943. He and Shanahan raised two daughters.
In his literary work, Knowles borrowed characters from Don Quixote, a famous novel by Miguel de Cervantes.