Born Jacqueline Lee Bouvier in Southampton, New York, she was the daughter of John Vernou Bouvier III, a Wall Street stockbroker, and his wife Janet Norton Lee. She had a younger sister, Caroline Lee Bouvier, born in 1933, and later known as Lee Radziwill.
Jacqueline Bouvier was of mostly Irish, Scottish, and English descent; her French paternal ancestry is distant, with her last French ancestor being Michel Bouvier, a Philadelphia-based cabinetmaker, merchant and real estate speculator who was her great-great–grandfather and a contemporary of Joseph Bonaparte and Stephen Girard. Both sides of her family made exaggerations about their heritage, with the Bouviers claiming descent from French nobility and the Lees declaring they were part of the "Virginia Lees."
She spent her early years between New York City and East Hampton, New York at the Bouvier family estate "Lasata". At a very early age she became an accomplished equestrienne, a sport that would remain a lifelong passion. As a child, she also enjoyed drawing, reading and lacrosse. This idyllic childhood came to an end when her parents divorced in 1940.
Her father never remarried. In 1942 her mother married second husband Standard Oil heir Hugh D. Auchincloss, Jr., and they had two children, Janet and James Auchincloss. Jacqueline and her sister Lee then lived with their mother's new family, dividing their time at their stepfather's two vast estates, "Merrywood", in McLean, Virginia, and "Hammersmith Farm", in Newport, Rhode Island. They remained close to their father, and visited him often in New York City, where he lived.