[4] During the First World War, Willson served as a senior officer with the Canadian Expeditionary Force.
He then served as Inspector of War Trophies on the Western Front and later in Palestine, in which role he was instrumental in the establishment of the Imperial War Museum: he believed that the museum's collections should reflect the detail of battle and involvement of ordinary soldiers at ground level.
[5] In early 1919 he was appointed Town Major (senior British officer) in Ypres as the city began to return to civilian life.
He repeatedly argued that the city should be left in ruins as a shrine to the war dead, but his high-handed actions towards this end, sometimes taken without official authority, eventually earned him a formal reprimand.
The family lived for a while at Quebec House, Westerham, Kent, the childhood home of James Wolfe (1727–1759).