As captain of the No. 1 gun on board the flagship U.S.S. Hartford, during action against rebel gunboats, the ram Tennessee and Fort Morgan in Mobile Bay, 5 August 1864. Although struck several times in the face by splinters, and with his gun disabled when a shell burst between the two forward 9-inch guns, killing and wounding 15 men, Fitzpatrick, within a few minutes, had the gun in working order again with new track, breeching, and side tackle, had sent the wounded below, cleared the area of other casualties, and was fighting his gun as before. He served as an inspiration to the members of his crew and contributed to the success of the action in which the Tennessee was captured.