Heigl was born in Washington, D.C., the daughter of Nancy, a personal manager, and Paul Heigl, a financial executive/acco auntant. Heigl has German and Irish ancestry, and was a member of The Church of Jesus Christ of Latter-day Saints. She is the youngest of four children (in addition to siblings Meg, Jason, and Holt). Heigl lived in Virginia and then Denver before her family settled in Connecticut, where they moved into a large, old Victorian-style farmhouse in the wealthy town of New Canaan, where she spent most of her childhood.
In 1986, her older brother Jason died of injuries suffered in a car accident, after being thrown from the back of a pickup truck while out for lunch with some of his high school classmates. Following his death, the family decided to donate his organs. Afterward, their parents converted to The Church of Jesus Christ of Latter-day Saints (prior to that her mother was Lutheran and her father Catholic). Heigl is now a strong proponent of organ donation. Although she is no longer a practicing Mormon, she remains positive on several aspects of the religion, and has expressed interest in returning to her faith.