Thorne was born in New York City. She was the daughter of Alice Smith Barry and Landon Ketchum Thorne, Jr. Her brothers are Landon Ketchum Thorne III of Beaufort, South Carolina and her twin brother David Thorne of Brookline, Massachusetts. She spent much of her childhood in Rome and attended the international school, Marymount in Rome while her brother David attended the Overseas School of Rome. Her father had been appointed to a diplomatic post and was publisher of the The Voice of the DAILY AMERICAN. She attended the Foxcroft School in Middleburg, Virginia. She took classes at the New York School of Interior Design and Radcliffe College.

Thorne met Kerry in 1963 at her family's estate in Bay Shore, New York on Long Island. Kerry was a Yale classmate of Thorne's twin brother David. Thorne married Kerry on May 23, 1970 and divorced on July 25, 1988, after a six-year separation. They have two children together, Alexandra Kerry (born 1973) and Vanessa Kerry (born 1976). During their marriage, Thorne began showing signs of depression and later wrote that she had at one time contemplated suicide. She overcame depression by 1990, and by all accounts the couple had an amicable relationship .