Brashares was born in Alexandria, Virginia and grew up in Chevy Chase, Maryland. She attended elementary and high school at the Sidwell Friends School in Washington D.C.. After studying philosophy at Barnard College, she worked as an editor for 17th Street Productions. 17th Street was acquired by Alloy Entertainment, and following the acquisition she worked briefly for Alloy. After leaving Alloy she wrote "The Sisterhood of the Traveling Pants" which became an international best seller that was followed with three more titles in the "Pants" series, the last of which, Forever in Blue, was released in January, 2007. The first book in the series was made into a film in 2005, and a second film based on the other three series titles was released in August, 2008. Brashares's first adult novel, The Last Summer (of You and Me) was released in 2007. She lives in New York with her artist husband, Jacob Collins, and children Nathaniel, Samuel, and Susannah.
Brashares mainly writes for young adults. Besides the 4 Sisterhood books, she has contributed two titles to the Techies non-fiction book series and published one novel. Delacorte Book Press has announced that she's writing a new book, 3 Willows: The Sisterhood Grows.