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. It 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 titles in the series was released in August, 2008. Brashares's first adult novel, The Last Summer (of You and Me) was released in 2007. A companion book to the Sisterhood series, 3 Willows: The Sisterhood Grows was published in 2009. A second novel for adults, My Name is Memory was published in 2010 and has been optioned for film. She lives in New York with her artist husband, Jacob Collins, and children Nathaniel, Samuel, and Susannah.