Clare was born Judith Rumelt, to American parents, in Tehran, Iran. Her parents are Elizabeth and Richard Rumelt, who are a business school professor and author, respectively. Her maternal grandfather was film producer Max Rosenberg. Clare is Jewish, and has described her family as "not religious".
As a child, Clare traveled frequently, spending time in Switzerland, England, and France. She returned to Los Angeles for high school, and from then on split her time between California and New York City, where she worked at various entertainment magazines and tabloids, including The Hollywood Reporter.
While living in Los Angeles Clare began writing fan fiction using the name Cassandra Claire. The Draco Trilogy, based on Harry Potter and The Very Secret Diaries, based on The Lord of the Rings were popular. However, she deleted her fan fiction from the Internet shortly before her first novel, The City of Bones, was published under the name Cassandra Clare.
She is also friends with author Holly Black, and their books occasionally overlap, Clare mentioning characters from Black's novels and vice versa, such as Val and Luis from Black's Valiant.
Clare is also credited by her publisher with creating the "City of Fallen Angels treatment" where a tangible "letter" from one character to another is attached to the back of physical copies of a book. The goal is to spur print book sales.
She currently resides in Amherst, Massachusetts, with her husband, Joshua Lewis, and three cats.
In 2004, Clare started working on her first-published novel, City of Bones, inspired by the urban landscape of Manhattan. City of Bones was released by Simon & Schuster in 2007 and is a contemporary fantasy story revolving around characters Clary Fray, Jace Wayland, and Simon Lewis, which became a New York Times bestseller upon its release. City of Ashes and City of Glass completed the first trilogy. A subsequent second trilogy contained three more books: City of Fallen Angels, City of Lost Souls, and City of Heavenly Fire.
There is a prequel trilogy called The Infernal Devices, which is set in the same universe as The Mortal Instruments, but set in the Victorian era. This consisting of three books: Clockwork Angel, published on August 31, 2010, Clockwork Prince, published on December 6, 2011, and Clockwork Princess, published on March 19, 2013.
A fourth trilogy set in this universe was announced in 2012, to be collectively known as The Dark Artifices. The new contemporary series will be set in Los Angeles, and follow female shadowhunter Emma Carstairs, who was introduced in City of Heavenly Fire. The first book, Lady Midnight, was released in March 2016 and the second, Lord of Shadows will be released in April 2017.
There are also two series of interconnected short stories set in this universe. The first is The Bane Chronicles, completed in 2014 and written with Sarah Rees Brennan and Maureen Johnson, and the second is the planned Tales from the Shadowhunter Academy, written with Brennan and Johnson as well as Robin Wasserman.
The first book in The Mortal Instruments was made into a film, The Mortal Instruments: City of Bones (2013), by Unique Features and Constantin Film. First-time writer Jessica Postigo wrote the screenplay. Lily Collins played Clary Fray and Jamie Campbell Bower played Jace Wayland.
After a disappointing box office performance, subsequent movies in the series were cancelled. A television adaptation of The Mortal Instruments called Shadowhunters began airing in January, 2016, and has been renewed for a second season.2010 Georgia Peach Book Awards for Teen Readers
Finalist for the Locus Award for Best First Novel of 2007
An American Library Association Teens Top Ten Award winner, 2008
2010 Georgia Peach Book Awards for Teen Readers
Winner of The 2010 Abraham Lincoln Illinois High School Book Award
Winner of the 2010 Pacific Northwest Library Association Young Reader’s Choice Award
A Texas TAYSHAS title 2010
Shortlisted for the 2010 Evergreen Young Adult Book Award
Shortlisted for The 2010 Colorado Blue Spruce Young Adult Book Award
Shortlisted for The North Carolina School Library Media Association Young Adult Book Award
Grand Canyon Reader Award Nominee (Teen) 2010
Iowa High School Book Award Nominee 2009-2010
North Carolina YA Book Award Nominee 2009-2010
New Hampshire Flume Teen Reader’s Choice Award Nominee, 2010
Nevada Young Readers’ Award Nominee 2010
Ohio Buckeye Teen Book Award Nominee, 2009
Oregon Young Adult Network Book Rave Reading List Title 2008
South Carolina Young Adult Book Award Nominee 2009-2010
Shortlisted for the Coventry Inspiration Book Awards
A 2009 ALA Teens Top Ten Title