Born in 1980, Llewellyn is the daughter of Sir David St Vincent "Dai" Llewellyn, 4th Baronet and Vanessa Hubbard. She has a younger sister, Arabella (born 1983). Llewellyn also has several half- and step-siblings, including actresses Gabriella Wilde and Isabella Calthorpe.
Trained at the London Academy of Music and Dramatic Art, Llewellyn appeared in supporting roles in several television series and films before portraying Mina Harker in the Showtime series Penny Dreadful in 2014 and 2015, and Isabel Danforth in the Lifetime series The Lizzie Borden Chronicles in 2015.
