Brewster was born in Panama City, Panama. She is the oldest of two daughters. Her mother, Maria João (née Leal de Sousa), is a former swimsuit model from Brazil who appeared on the 1978 cover of Sports Illustrated, and her father, Alden Brewster, is an American investment banker. Her paternal grandfather, Kingman Brewster, Jr. , was an educator, president of Yale University (1963–77), and the United States Ambassador to the United Kingdom (1977–81). Brewster is a direct descendant of Mayflower passengers William Brewster and Edward Doty. She left Panama when she was two months old, relocating to London, where her younger sister was born. She would spend six years, before moving to her mother's native city of Rio de Janeiro, where she learned to speak Portuguese fluently. She left Brazil at the age of 10, settling in Manhattan, New York City, where she studied at the Convent of the Sacred Heart and graduated from the Professional Children's School. Brewster graduated from Yale University with a B. A. in English in 2003.