Shaking down in her new configuration, the submarine performed a "first" for submarines on 6 November 1954, when she snorkeled in the Mississippi River at New Orleans, Louisiana, from the Industrial Canal to the foot of Canal Street. With SubRon 4, the ship was based at Key West, Florida, and visited ports in the Caribbean Sea before entering the Charleston Naval Shipyard in February 1956 for overhaul. Upon completion of this work, the submarine was deployed to the Mediterranean Sea for a tour with the Sixth Fleet before returning to Key West in March 1957. While with SubRon 4, "Thornback" participated in operations supporting the Operational Development Force, the Fleet Sonar School, and the Fleet Training Unit at Guantanamo Bay, Cuba.