On September 10, 1951, he was wounded in action. On September 12, he was killed in action near Songnap-yong, South Korea, on Hill 673. Mausert had run through an enemy mined area under fire to bring to safety two seriously wounded Marines and suffered a head wound. Refusing evacuation, he led his squad and platoon in a bayonet charge up the hill by his rifle company against enemy bunkers and emplacements and was wounded again. He destroyed two machinegun positions with hand grenades. He was killed spearheading his platoon's attack on the remaining North Korean bunkers on the hill which they then overran.