Popplewell, oldest of three children, is the daughter of the High Court judge Andrew Popplewell and Dr. Debra Lomas, a dermatologist who studied at Newnham College Cambridge. She was born in London. Her siblings are actress Lulu Popplewell, who starred as Daisy in Love Actually, and Freddie Popplewell, who starred as Michael Darling in the film Peter Pan. Her paternal grandfather, Sir Oliver Popplewell, is a former judge, and her uncle is former cricketer Nigel Popplewell. She attended North London Collegiate School and was Head Girl of 2006–07. She was admitted to Oxford University in 2007 where she studied English Language and Literature at Magdalen College.