[1] Prettejohn was born to mother Claire, a lawyer, and father Nick, the chairman of TSB Bank, and grew up in Kensington, London.
[3] Prettejohn made her television debut in the second season of the Starz series Counterpart as a young version of Olivia Williams' character Emily.
[4] In August 2019, Prettejohn joined the cast of the Joss Whedon series The Nevers on HBO as Myrtle Hapslich.
[7] In so doing, she became the fifth actress after Claire Foy, Olivia Colman, Imelda Staunton, and Verity Russell to play Elizabeth on the series.
[9][10] She has an upcoming role in the BBC One adaptation of Hilary Mantel's Wolf Hall: The Mirror and the Light as Mary FitzRoy, Duchess of Richmond and Somerset.