He was a future field marshal and was created Viscount Alexander of Tunis in 1946 and Earl Alexander of Tunis in 1952.
Owing to her husband's career, the family moved frequently, and they lived in 36 homes during their marriage, during which time she held several prominent roles.
In 1946, on her husband's appointment as Governor General of Canada, she became Vice-regal consort, and Châtelaine of Rideau Hall.
In recognition of this she was made a Dame Grand Cross of the Order of the British Empire in 1954.She held the office of Justice of the Peace () for Berkshire in 1956–75.
In 1970, while living at Windsor Great Park, she was appointed a Deputy Lord Lieutenant of Berkshire, one of the first women to hold the role of JP.Lady Margaret was active at the Women's Voluntary Service during the Second World War, working for the apartment in charge of evacuation of small children.
She was Vice-Chairman of the WVS 1952–53 and served on the board of the English-Speaking Union from 1953–1971.
Between 1960–65, she established the Winston Churchill Memorial Trust at the ESU.
She served as chairman of the Juvenile Court of Windsor County Division, and set up a probation hostel for young men in the area, visited the hostel weekly from 1965-1976.
She died Bracknell, Berkshire in 1977, aged 71.