After arriving in France in February 1918, Thompson was assigned to the Intelligence staff section (G-2) of the American Expeditionary Force headquarters. From July to September 1918, Thompson was assistant chief of staff for Intelligence on the staff of First United States Army. He participated in both the Aisne-Marne offensive and Battle of Saint-Mihiel. From September 1918 to April 1919, Thompson served as G-2 for Second U.S. Army, which included post-war occupation of the Toul sector. He was promoted to temporary lieutenant colonel in October 1918.