Photograph portrait of Princess Mary, half length, looking to her right, in the uniform of the Auxiliary Territorial Service (ATS) decorated with ribbon bars. It is possible that this uniform was especially designed and commissioned by Norman Hartnell, as part of the new uniform commissioned in 1941, at a time of great material shortage.

The ATS was the women's branch of the British Army during the Second World War. Princess Mary became Chief Controller in May 1941. Later that year in December, Parliament passed the National Service Act, which called up unmarried women between 20 and 30 years old to join one of the auxiliary services, including the ATS, Women's Royal Naval Service (WRNS), the Women's Auxiliary Air Force (WAAF), the Women's Transport Service, Women's Voluntary Service and the Women's Land Army. Married women were also later called up, except for those who were pregnant or with small children.