Doyen was launched as transport AP-2 on 9 July 1942 by Consolidated Steel of Los Angeles, California, under a Maritime Commission contract; sponsored by Miss F. D. Johnson, granddaughter of BGen Doyen; reclassified APA-1, 1 February 1943; acquired by the Navy 20 April 1943 and converted at Bethlehem Steel of San Pedro, California; and commissioned 22 May 1943.
She arrived at Wellington 24 October and on 1 November sailed with men of the 2nd Marine Division for the assault on the Gilbert Islands. From 20 to 24 November she landed her troops on Tarawa and embarked casualties under attack from enemy shore batteries and torpedo planes. After bringing her passengers to Pearl Harbor, Doyen returned to the west coast for training duty, arriving at San Diego on 18 December.
Invasions of Kwajalein, Saipan and Guam
Doyen got underway 13 January 1944 for the invasion of Kwajalein, landing her troops the last day of the month and receiving casualties and prisoners of war for transportation to Pearl Harbor where she arrived on 15 February. She remained in the Hawaiian Islands on training duty until 30 May when she sailed for Eniwetok thence to the invasion of Saipan from 16 to 22 June and Guam from 22 to 28 July.
Invasion of Leyte
Doyen arrived at Manus 3 October 1944 to join up for the invasion of the Philippines. She put her troops ashore in the assault in Leyte Gulf on 20 and 21 October, then sailed to Humboldt Bay, New Guinea, for reinforcements whom she landed in Leyte Gulf on 18 November.
Invasions of Lingayen Gulf and Iwo Jima
Returning to Manus on 24 November Doyen loaded Army troops at Cape Torokina, Bougainville, and trained them at Huon Gulf, New Guinea, for the amphibious assault at Lingayen Gulf on 9 and 10 January 1945. After repairs at Ulithi, she loaded cargo and embarked Marines at Guam. On 16 February she sailed for the initial assault on Iwo Jima on 19 February. She lay off the island to receive casualties whom she landed at Saipan on 9 March, then carried Seabees to Guam before arriving at Nouméa on 23 March for repairs and training.
Transport missions
Doyen left Nouméa 3 May 1945 to carry troops to San Pedro Bay, Leyte, then sailed to Guam to embark patients, with whom she arrived at San Diego on 17 June. After a complete overhaul, she carried troops and returning veterans between the west coast and Pearl Harbor from 28 September to 30 December.
Decommissioning and fate
On 4 January 1946 she put out from Seattle for the east coast, arriving at New York on 4 February. Doyen was decommissioned on 22 March 1946, and transferred to the Maritime Commission for disposal on 26 June 1946.
The Doyen received a reprieve from the scrap yard as it was renamed the Bay State, and served as a training vessel, dormitory and classroom for the Massachusetts Maritime Academy in Buzzards Bay from 1957 to 1972 under the name USTS Bay State. The story of the ship's service in the Pacific during World War II as well as later as the Bay State is chronicled by Lt. Cdr. Lawrence A. Marsden (SC) USN (ret) in the book "Gemini Ship," published 2002 by Infinity Press.