She has English, Irish, Scottish, Welsh, and distant Dutch, ancestry.
Witherspoon was born on March 22, 1976 at Southern Baptist Hospital, in New Orleans, Louisiana, while her father, Dr. John Draper Witherspoon, was a student at Tulane University medical school. Dr. Witherspoon was born in Georgia and served as a lieutenant colonel in the United States Army Reserve. He was in private practice as an otolaryngologist until 2012. Her mother, Dr. Mary Elizabeth "Betty" (née Reese), is from Harriman, Tennessee. Dr. Betty Witherspoon earned five degrees in total, including a Ph.D in pediatric nursing. She became a professor of nursing at Vanderbilt University. Witherspoon has claimed descent from Scottish-born John Witherspoon, who signed the United States Declaration of Independence; however, this claim has not been verified by the Society of the Descendants of the Signers of the Declaration of Independence genealogists. Her parents are still legally married, although they separated in 1996.
Because Witherspoon's father worked for the U.S. military in Wiesbaden, Germany, she lived there for four years as a little girl. After returning to the U.S., she spent her childhood in Nashville, Tennessee. Witherspoon was raised as an Episcopalian, and has said she is proud of the "definitive Southern upbringing" which she received. She said that it gave her "a sense of family and tradition" and taught her about "being conscientious about people's feelings, being polite, being responsible and never taking for granted what you have in your life."