Milicent had a reputation as a formidable security officer and specialised in international communism throughout her service, advising on the threat posed by the Soviet bloc to Britain during the Cold War. After the war Milicent spent some time working with British authorities in the Middle East, guiding them on how to overcome Soviet subversion. By 1949 she was recognised as a leading expert on the matter for her unparalled knowledge and was promoted to Assistant Director of MI5 in 1953. Milicent is thought to be the inspiration for the character Connie Sachs from John le Carré’s novels Tinker, Tailor, Soldier, Spy, The Honourable Schoolboy and Smiley’s People.  

Although most of the details about her career remain shrouded in secrecy, one of her most significant cases involved an investigation to decipher the infamous Zinoviev Letter of 1924, a document which detailed Soviet plans to encourage a revolution from British workers and called on them to strike. The letter was leaked amid a divisive general election and is thought to have contributed to the collapse of the Labour government at the time. Although the true origins of the letter are still unconfirmed, Milicent’s work contributed to further investigations which concluded that it was most likely the result of forgery crafted by anti-Soviet emigrants living in Latvia.