Karen Hesse was born and raised in Baltimore, Maryland, began college at nearby Towson State University, then transferred on a work study program to the University of Maryland at College Park. There, she worked in the McKelden library, shelving and cataloging books, and helping reference patrons. After graduation she held jobs as a benefit coordinator for the University of Maryland, a librarian, teacher, advertising secretary for Country Journal magazine, typesetter, and proofreader. She married her college boyfriend, Randy Hesse, and now lives with him and their two daughters, Kate and Rachal, in Brattleboro, Vermont. She says that as a typesetter in 1980 she realized that she probably had a talent for writing children's books, but she did not publish her first book for young people, Wish on a Unicorn, until 1991. Since then, she has published at least one book every year—some for young adult readers and others for younger readers—and has quickly gained a reputation as an important writer for young adults. Among her numerous awards, Letters from Rifka (1992) was awarded the Christopher Medal and the Horn Book Fanfare in 1992; The Music of Dolphins (1996) was named Best Book by both Publishers Weekly and School Library Journal for 1996; and Out of The Dust won the prestigious Newbery Award for 1998. Her latest books, Just Juice (1998) and Come On, Rain (1999) are illustrated books for younger readers.
She has written of herself as thin and pasty, friendly but alone even though she was always surrounded by people. She read a great deal, and at the age of eleven or twelve discovered John Hersey's Hiroshima, which had a profound effect on her as she understood the horror and dignity with which the Japanese people of that island endured the blast of the first atomic bomb. She writes, "If more books for children had existed at that time with real issues, if I had seen characters survive the engulfing engine of reality, I don't think I would have felt so lonely, so isolated. I write now for children like the child I was, to show young readers that they are not alone in this world. My hope is to help them through hard times, to present characters who survive ordeals and grow as a result of them."