Cushman was born on Oct. 4, 1941, in Chicago, Ill., but moved to southern California at age 11. Although she enjoyed reading and writing in her youth, she never seriously considered becoming a children's writer. She studied English and Greek at Stanford University and received her bachelor's degree in 1963. After graduation, she held a variety of jobs, became a wife and mother, and later pursued a graduate degree in human behavior at United States International University. After earning a master's degree in museum studies at John F. Kennedy University in 1986, she joined the institution's staff as a teacher and editor.

Although she often told her family about ideas she had for stories, Cushman did not begin putting her thoughts down on paper until she was almost 50 years old. Her first book, ‘Catherine, Called Birdy', was published in 1994. The book was set in the Middle Ages, and Cushman meticulously researched the period using a range of primary sources. She also studied the historical fiction of many well-known children's writers in order to get a better feel for writing for young adults. Written in diary format, the book chronicles the daily life of a teenager living in an English manor house with a mother intent on making her a proper lady and a father constantly attempting to marry her off. The coming-of-age story was chosen as a Newbery Honor Book in 1995 and won the Carl Sandburg Award for Children's Literature.

Cushman received the Newbery Medal the following year for ‘The Midwife's Apprentice' (1995). Although also set in the Middle Ages and involving a strong female lead character, this book centers on a homeless girl who gains purpose and confidence when she becomes the helper of a midwife who discovers her sleeping in a dung heap.

Cushman's third novel, ‘The Ballad of Lucy Whipple' (1996), takes place during the period of the California gold rush. The heroine learns about the true meaning of home when her family leaves Massachusetts for a new life in the West.

Cushman was honored by numerous organizations, including the Society for Children's Book Writers and Illustrators, the Parent's Choice Foundation, and the American Booksellers Association. She frequently visited schools to talk to youngsters about her books and career.