Benson became a subaltern with the 9th Queen's Royal Lancers the following year and was appointed aide-de-camp to the Viceroy of India in 1913. He returned to his regiment when the First World War broke out and served in France in the battles of Aisne, Ypres and Messines; for his conduct in the latter, he was awarded the Military Cross. In 1915, he was gassed and wounded during the Second Battle of Ypres and repatriated to England for treatment. When he had sufficiently recovered, he was appointed as liaison officer to the French Minister of Marine, but he was unofficially a representative of the head of the British secret service. After serving in Ireland during the Easter Rising (1916), he returned to France as liaison officer firstly with General Franchet D'Esperay and then with Marshal Petain; the French government awarded him the "Croix de Guerre" and appointed him a member of the "Légion d'honneur". With the war over, he became Chief of the British Mission at the Paris Peace Conference of 1919. After promotion to Major in 1920, Benson became Military Secretary to the Governor of Bombay, and assisted with organising the Prince of Wales's 1922 tour. He resigned from the Army thereafter, but was placed on a secret mission by the Prime Minister David Lloyd George to trade with the Soviet Union. He sold tea and other supplies to the value of £10,000, which he smuggled out in his boots.