When the lead platoon was suddenly taken under devastating enemy fire by automatic weapons and snipers from concealed positions during a reconnaissance-in-force operation in the Cam Lo River Valley, Quang Tri Province, Petty Officer Mack immediately began to administer aid to the wounded.

Upon being informed that two seriously wounded Marines needed medical aid, but couldn't be evacuated due to intense and accurate enemy fire, he unhesitatingly moved over two hundred meters down the fire-swept road to reach the casualties. With bullets striking the ground near him. he nonetheless succeeded in administering lifesaving aid to the wounded men and remained with them until they were evacuated.

Before moving back to a covered position in preparation for an air strike, Petty Officer Mack once again exposed himself to the intense enemy fire to assist a wounded Marine in the advance element who needed medical aid prior to being evacuated. While rendering first aid to the wounded man, Petty Officer Mack himself was mortally wounded.

By his extraordinary initiative, inspiring valor and unswerving devotion to duty, he was directly responsible for saving the lives of three fellow servicemen. His heroic actions were in keeping with the highest traditions of the United States Navel Service.