'Messines Offensive, 7th to 9th June 1917. During the attack on Messines the Officer in command of No 8 Platoon was severely wounded and Sergt McMahon took command of the platoon and ably led it to the line on which the Platoon was to consolidate. During a long and severe bombardment of our consolidated positions this NCO by his coolness and energy set an excellent example to the men. All Platoon Officers were wounded, and Sergt McMahon ably consolidated his sector, and got into touch with the troops on our right. He showed great devotion to duty and from the commencement of the attack until our relief 60 hours later performed excellent work.'
Source: London Gazette
Date: 16 August 1917