She was originally assigned to duty on Lake Superior and served there until transferred to the operational control of the United States Navy. Transferred to the Navy by the Executive order of 16 April 1917 which placed the Lighthouse Service under the control of the Navy Department, Amaranth was assigned to the 9th Naval District, but continued to serve much as she had done before the war. Following the armistice, she was returned to the jurisdiction of the Department of Commerce with the rest of the Lighthouse Service under an Executive order of 1 July 1919.
On the morning of 23 July 1920, while supplying Passage Island Light Station, Lake Superior, Michigan, the Amaranth struck the rocks under water at the extreme southwesterly point of the island, breaking the shoe and rudder, with the total loss of the latter. Repairs were made by the Port Arthur Shipbuilding Company, Port Arthur, Ontario. She also received new boilers and then returned to service.
In 1939 President Franklin D. Roosevelt merged the Lighthouse Service into the United States Coast Guard which, on 1 November 1941, was ordered to "...operate as a part of the Navy." Now redesignated USCGC Amaranth (WAGL-201), the ship was stationed at Duluth, Minnesota, throughout World War II, and maintained navigational aids on Lake Superior. Following the return of peace, she was decommissioned on 29 September 1945 and sold on 19 October 1946. She served as the privately owned freighter South Wind until being laid up in 1954.