Aitken was born in Dublin, Ireland, the daughter of Sir William Aitken, a Conservative MP, and Penelope Aitken, whose father was John Maffey, 1st Baron Rugby. Her grandfather was the UK Representative to Ireland (1939–49). She is a great-niece of newspaper magnate and war-time minister Lord Beaverbrook, and sister to former Conservative cabinet minister Jonathan Aitken. She attended Riddlesworth Hall Preparatory School in Norfolk, Sherborne School for Girls in Dorset and St Anne's College, Oxford, where she graduated with a degree in English Language and Literature.