During World War I, Thomson served in the 143rd Field Artillery Regiment, known informally at the time as the "Mary Pickford Regiment". Thomson joined the 143rd in Arcadia, California as a U. S. Army chaplain. While playing football, he broke his leg. Movie star Mary Pickford visited the patients in the hospital ward with her friend, screenwriter Frances Marion. Thomson and Marion agreed to marry after the war was over. The 143rd were sent to France in August 1918, but did not see any action before the armistice on November 11, 1918.