Mullens was assigned to Company C led by Captain John H. Miller. On D-Day, June 6th,1944, Mullens was to land on Omaha Beach with the 2nd Rangers Battalion's Charlie Company. He was shaking as he was soaked with water while in the craft behind his comrade, Private Isaac. Just as the craft landed, he quickly did the sign of the cross, evidently praying.
Before his company's landing craft could reach the shore, it came under fierce fire from the German MG-42s in the coastal bunkers. Captain Miller shouted the order for his company to jump into the water to avoid the barrage. Mullens was killed when he was shot in the head.