Following Churchill's call to form a "butcher and bolt" raiding force following the evacuation of Dunkirk, Mayne volunteered for the newly formed No. 11 (Scottish) Commando. He first saw action in June 1941 as a second-lieutenant with 11 Commando during the Syria–Lebanon Campaign. Mayne successfully led a section of men during the Litani River operation in Lebanon against Vichy French Forces. The operation was commanded by Major Dick Pedder, Highland Light Infantry, who was killed in action. Mayne played a distinguished part in the raid, for which he was awarded a mention in despatches.