Her father was a British army officer of Dublin and Ballingaddy, County Limerick, and served with distinction in France and the Balkans during World War I.
O'Donnell attended St. Mary's Convent in Ascot, Berkshire from 1929 to 1935, going on to study the history of art, entering the Courtauld Institute in 1937.
She was appointed to the Special Operations Executive (SOE) in September 1940 to work in the headquarters of the Czech section, and given the rank of officer.
She left the SOE in December 1944 to work with the Red Cross, caring for survivors of concentration camps.
O'Donnell became a member of the Special Forces Club in London, counting among her friends numerous prominent immigrants including, Rudolf Wittkower, Ernst Gombrich, and Johannes Wilde.
[1] O'Donnell noticed that no university or other institution ran a course that covered all elements of the fine and decorative arts.