Cooper joined the
Women's Royal Naval Service ("Wrens") in September 1941. She was trained at
Westfield College, London, and expected to become a cook but part way through her training a request was received, said to be from
Winston Churchill himself, for volunteers to do unspecified secret work. Cooper and most of her class accepted and quickly found themselves at the signals interception and decoding base of
Bletchley Park in central England and not the naval base they had expected. At first she worked in Hut 11 on the
bombes that decoded intercepted messages but in late 1942 she was moved to
Stanmore where backup bombes were being set up as a precaution against the destruction of the originals by enemy bombing. She then became assistant to
Frank Birch, head of the naval section in Hut 4, which was known as the "U-boat room". Cooper would also regularly liaise with the Admiralty's Operational Intelligence Centre in London. She was promoted to petty officer, and then commissioned as a third officer, after which in April 1944 she was sent to Plymouth to work at the underground base there at
Mount Wise, liaising between Bletchley and Plymouth on the movements of U-boats. She never talked about her secret work until after the nature of the code-breaking operation was revealed by
F.W. Winterbotham in
The Ultra Secret (1974). ==Personal life==