Sandell was born in Wilmington, Delaware to physical chemist Lionel Sandell and teacher Nancy Sandell.

Before she published her first book, Sandell interned in Jerusalem, Israel at the news magazine The Jerusalem Report. The experience informed Sandell's first novel, The Weight of the Sky, published by Viking Children's Books in 2006.

In 1999, Sandell received a B.A. in Medieval and Renaissance Literature from the University of Pennsylvania in Philadelphia, Pennsylvania. Her honors thesis on Sir Thomas Malory's treatment of Lancelot in Le Morte d'Arthur provided the inspiration for her second novel, Song of the Sparrow.

Sandell moved to New York City in 2000, and began work as a children's book editor. She continues to write for young adults, and wrote a short story titled "See Me" for the anthology 21 Proms, which was published by Point in 2007.