Following shakedown, Northampton reported for duty to Commander Operational Development Forces, Atlantic Fleet. For seven months she conducted extensive tests of her new equipment. Evaluation completed in September 1954 and she reverted to the operational control of Commander Battleship Cruiser Force, Atlantic Fleet. She next demonstrated her capabilities as a tactical Command Ship by serving as flagship, first for Commander Amphibious Force, Atlantic Fleet (October–November 1954) and then for Commander 6th Fleet (December 1954–March 1955). Between 1 September and 22 October she served as flagship for Commander Strike Force, Atlantic, a position she was to hold frequently over the next fifteen years.