She was reading German at University College, London, when war broke out, and decided to break off her studies and become a nurse; but she was told that the country could make more use of her as a German linguist. “So I thought, great,” she recalled. “This is going to be an interesting job, Mata Hari, seducing Prussian officers. But I don’t think either my legs or my German were good enough because they sent me to the Government Code & Cipher School.”
And a good thing they did, too, because she, one other young woman named Margaret Rock, and Dilly Knox were responsible for cracking the code of the Abwehr Enigma machine, which in turn allowed for the success of the D-Day landings. Mavis is the one who "broke a message on the link between Belgrade and Berlin, allowing the reconstruction of one of the rotors." She was 20 at the time.