In 1941 he joined the Royal Air Force, and by the winter was flying combat with No. 121 (Eagle) Squadron, composed of American volunteers. Pilot Officer Mahon damaged three German fighters and two Focke-Wulf Fw 190s in April and a Messerschmitt Bf 109 in May, before scoring his first confirmed victories of two Fw 190s, downed in the Saint-Omer area, on June 8, 1942. On July 31 he destroyed two more Fw 190s over the Berck-Sur-Mer area and became an ace while covering the commando raid on Dieppe, August 19, 1942, his 98th fighter pilot mission. At 08:30, flying his Spitfire Vb, nicknamed "Barry", Mahon shot down an Fw 190 and another Fw 190 before his plane was hit and he was forced to bail out of the burning aircraft. Landing in the sea, he was taken prisoner. Although Mahon filed a claim for shooting down two Germans that day, he was only given credit for one after the war. Thus, for a tally-record of five confirmed, two probables and three damaged Mahon received the British Distinguished Flying Cross in 1985.