In 1914, Coningsby Dawson went to Ottawa, saw Sir Sam Hughes, and was offered a commission in the Canadian Field Artillery on the completion of his training at the Royal Military College of Canada, at Kingston, Ontario. "His long training at Kingston had been very severe. It included besides the various classes which he attended a great deal of hard exercise, long rides or foot marches over frozen roads before breakfast, and so forth." In July 1916 he was selected, with twenty-four other officers, for immediate service in France. His younger brothers enlisted in the Naval Patrol, then being recruited in Canada by Commander Armstrong.

Lieutenant Coningsby Dawson joined the Canadian Army at the front in 1916, and continued in service until the end of World War I. He served in the Somme battlefield at Albert, at Thiepval, at Courcelette, and at the taking of the Regina trench. After having been wounded he came twice to the United States (1917, 1918) on lecture tours. In 1918, he investigated for the British Ministry of Information, American military preparedness in France.

Carry On: Letters in Wartime, by Lieutenant Coningsby Dawson, Novelist and Soldier, Canadian Field Artillery 1917 includes: "By the time this reaches you I'll be in the line again, but for the present I'm undergoing a special course of training. You can't hear the most distant sound of guns, and if it wasn't for the pressure of study, similar to that at Kingston, one would be very rested." February 4, 1917.