Cassandra Clare was born to American parents in Teheran, Iran and spent much of her childhood travelling the world with her family, including one trek through the Himalayas as a toddler where she spent a month living in her father's backpack. She lived in France, England and Switzerland before she was ten years old. Since her family moved around so much she found familiarity in books and went everywhere with a book under her arm. Her favorites included the work of the Brontë sisters, the Chronicles of Narnia, Tolkien, Lloyd Alexander's Prydain Chronicles and Susan Cooper's Dark is Rising series. She spent her high school years in Los Angeles where she used to write stories to amuse her classmates, including an epic novel called "The Beautiful Cassandra" based on the eponymous Jane Austen short story (and from which she later took her current pen name).

After college, Cassie lived in Los Angeles and New York where she worked at various entertainment magazines and even some rather suspect tabloids where she reported on Brad and Angelina's world travels and Britney Spears' wardrobe malfunctions. She started working on her YA novel, City of Bones, in 2004, inspired by the urban landscape of Manhattan, her favourite city. She turned to writing fantasy fiction full time in 2006 and hopes never to have to write about Paris Hilton again.

Cassie's first professional writing sale was a short story called "The Girl's Guide to Defeating the Dark Lord" in an anthology of humor fantasy. She also has a short story published in the anthology So Fey, from Haworth Press, and another in the anthology Magic in the Mirrorstone from Mirrorstone Books. Cassie hates working at home alone because she always gets distracted reality TV shows and the antics of her two cats, so she usually sets out to write in local coffee shops and restaurants. She likes to work in the company of her friend Holly Black, who sees that she sticks to her deadlines.

City of Bones is her first novel.