Blunt was born on February 23, 1983, in Roehampton, South West London, England, the second of four children in the family of Joanna Mackie, a former actress, and teacher, and Oliver Simon Peter Blunt, a barrister. She has three siblings: Felicity (Sister), Sebastian (Brother) & Suzanna (Sister). Her grandfather was Major General Peter Blunt, and her uncle is Member of Parliament Crispin Blunt. The famous actress comes from a traditional British family. Her close relatives served in the national army.