Guinness is the great-granddaughter of Diana Mitford (The Honourable Lady Mosley), who was one of the Mitford sisters, and her first husband Bryan Guinness, later the 2nd Lord Moyne. Her paternal grandfather, the Hon. Desmond Guinness, is a conservationist specialising in Georgian and classical architecture, while her paternal grandmother, Mariga Guinness, was born Marie-Gabrielle, Princess of Urach. She is 2260th in the Line of Succession to the British throne.