The cargo ship Niel Maersk was built by Odense Staalskibsværft A/S in Odense, Denmark in 1931, in July 1941 the ship was seized at Baltimore and turned over to the War Shipping Administration. In August 1941 the ship placed under the management of the Isthmian Steamship Company and renamed Johnstown.

On June 5, 1942 the ship was sailing alone returning to the USA from Karachi and was en route to make a stop at Lourenço Marques, Mozambique when the Japanese submarine I-20, which had just made a successful attack on British ships a Diego Suarez, torpedoed and sank the ship in the Mozambique Channel.

Three torpedoes were fired at the ship, the first one passed astern and exploded, but the other two hit her. The first in the engine room and the second in the stern. According to survivors she went down quickly and was abandoned after the first torpedo strike. All forty-one men got off the ship, two others had been killed by the explosions, but three of the survivors did not make in into the lifeboats. The three, two U.S. Armed Guard and one seaman, jumped into the water and had to board a raft. The two lifeboats made for land and arrived at Durban, South Africa on June 9. For some unknown reason they had not located the three men in the water before they left the area.

Their rescue came from a radio report made by the ship Viga, which sent out a signal stating that an unknown ship had exploded four miles astern of her position. Apparently no distress signal had ben sent out by Johnstown. Viga's signal was picked up by the ship Tasmania and her master altered his course to the scene of the stricken vessel. The attack had taken place at 0600, but the three men were not located until 1600 the next day. They were landed at Aden on June 16.

I-20 was one of the submarines which launched midget subs during the attack on Pearl Harbor and it was a midget attack at Diego Suarez which damaged HMS Ramillies and sank the tanker British Loyalty. In September 1943 I-20 failed to return from a sortie off Espiritu Santo, she was last heard from on Aug. 31. It is presumed she was sunk by a U.S. destroyer attack in early September.