On July 18, 1863, the soldiers of Carney's infantry regiment, the 54th Massachusetts colored infantry unit led the attack on Fort Wagner (in South Carolina). During the battle, the unit's color guard was shot. Carney, who was just a few feet away, saw the dying man stumble, and he scrambled to catch the falling flag.

Despite suffering several serious gunshot wounds himself, Carney kept the symbol of the Union (the flag) held high as he crawled up the hill to the walls of Fort Wagner, urging his fellow troops to follow him. He planted the flag in the sand at the base of the fort and held it upright until his near-lifeless body was rescued.

Carney lost a lot of blood and nearly lost his life, but not once did he allow the flag to touch the ground. His heroics inspired other soldiers that day and were crucial to the North securing victory at Fort Wagner. Carney was promoted to the rank of sergeant for his actions.