Sarah is the daughter of the romance writer Elissa Ambrose. Her parents are divorced, and she has one sister, Aviva Mlynowski, and an older stepsister. 
She graduated with an Honors degree in English literature from McGill University. Later, she moved to Toronto to work for Harlequin Enterprises Ltd. She used her romance publishing experiences to fuel her first novel Milkrun, which has since been published in 16 countries, selling over 600,000 copies around the globe. After her second novel Fishbowl was published, she moved to New York City to write full-time. She is best known for her New York Times best-selling series Whatever After.
Sarah married her husband Todd in 2004, and together they have two daughters.