Mayer claimed his first aerial victory following his tour as an instructor on 18 August during the Battle of the Falaise Pocket. A pocket had been formed around Falaise, Calvados, in which the German Army Group B, with the 7th Army and the Fifth Panzer Army (formerly ) were encircled by the Western Allies following the Normandy landings on 6 June. Flying multiple missions in support of the encircled German forces, Mayer shot down a North American P-51 Mustang from the No. 315 Polish Fighter Squadron and a P-47 fighter near Beauvais. On 25 August, the day Paris was liberated, the USAAF Ninth Air Force attempted to eliminate the reaming German fighter forces in France. Defending against this attack, Mayer shot down a 354th Fighter Group P-51 near Saint-Quentin.