In June 1917, Conner was selected by General John J. Pershing to be a member of the operations section (G3) for the American Expeditionary Force staff in France. He was promoted to lieutenant colonel on May 15, 1917 and to temporary colonel on August 5. In November Conner was selected as Pershing's Assistant Chief of Staff for Operations (G3); his subordinates included John McAuley Palmer and George C. Marshall. Conner developed an immense respect for both, and later referred to Marshall as the ideal soldier and a military genius.