On the outbreak of the First World War, Symonds left his medical studies and joined the British Army, serving as a despatch rider in the motorcycle section of the Royal Engineers. Attached to the 1st Division (sources are unclear if this was 1st Infantry Division or 1st Cavalry Division), he saw action at the Retreat from Mons, then in the battles of Marne and Aisne. He was wounded at the start of the Race to the Sea in September 1914, and was awarded the Médaille militaire, and then returned to his medical studies at Guy's.