Cimarron cleared Houston 31 May 1939 for Pearl Harbor, arriving 21 July. She transported oil between west coast ports and Pearl Harbor, making 13 such voyages until she sailed for the east coast on 19 August 1940. After repairs and alterations, she began oil runs on the east coast, principally between Baton Rouge and Norfolk, until August 1941, when she took part in amphibious operations. From 5 to 16 September she put to sea with a transport convoy bound for Iceland, and voyaged north again 12 October to 5 November to refuel ships at Placentia Bay. On 15 November, she joined a convoy at Trinidad bound with reinforcements for Singapore, but was detached from the convoy 9 December at Capetown, South Africa. Returning to Trinidad 31 December, she operated from Brazilian ports to Iceland until 4 March 1942, when she cleared Norfolk for San Francisco.

Cimarron reached San Francisco 1 April 1942 and sailed the next day with the task force bound for the first air raid on Tokyo 18 April. One of two oilers with the force, she fueled the Fleet at sea before and after the raid, and returned to Pearl Harbor 25 April. She sailed 29 April, bound to join the force soon to join battle with the Japanese in the Coral Sea, but arrived after the battle concluded, fueled destroyers at Noumea, and returned to Pearl Harbor 26 May. She cleared Pearl Harbor 28 May to fuel the force which defeated the Japanese in the Battle of Midway and returned 12 June, departing 7 July to support the operation in the Solomon Islands. Using Noumea as her principal base, Cimarron occasionally reloaded at Suva and Efate. After repairs at San Francisco in November, she sailed for the forward area 18 December. She operated again out of Noumea supporting the final stages of the Guadalcanal action, then fueled out of Efate, carried cargo to Sydney, Australia, and returned to fueling at Dumbea Bay in support of the occupation of New Georgia. She returned to San Francisco (in July 1943), and then made two trips from the west coast to Pearl Harbor.

Cimarron departed Pearl Harbor 29 September 1943 with the force which raided Wake Island on 5 and 6 October, and returned to Pearl Harbor 16 October. She sailed once again 14 November to fuel in support of the Gilbert Islands campaign, returning 1 December, and sailed to San Pedro to reload 12 December to 4 January 1944. Clearing Pearl Harbor 13 January, she supported the Marshalls operation and the February attacks on Truk from Majuro until 6 June; the Marianas operation from Eniwetok until 26 August; and the Palau Islands operation from Ulithi.

After a stateside overhaul from October through December 1944, Cimarron arrived at Ulithi 26 December 1944. From 27 December to 21 January 1945 she sailed to fuel the task force launching air attacks on Indo-China and Philippine targets as part of the Luzon invasion, and put to sea once more from 8 February to 22 March for air raids on the Japanese home islands and the invasion of Iwo Jima. From 26 March to 23 May she sailed from Ulithi to fuel ships engaging in the Okinawa operation, and from 3 June shuttled between Ulithi and the areas from which the mighty carrier task forces launched the final series of raids upon the heartland of Japan. Ulithi remained her base as she supported the occupation until 10 September, when she anchored in Tokyo Bay. Operations in the Far East continued until 4 February 1946, when she arrived at San Pedro, Calif., for overhaul.

Between July 1946 and June 1950, Cimarron ferried oil from the Persian Gulf to naval bases in the Marianas and Marshalls, occasionally continuing on to the west coast. Her first tour of duty in the Korean war, from 6 July 1950 to 3 June 1951, found her fueling ships of the Taiwan Patrol at Okinawa, amphibious ships at Kobe, and operating from Sasebo to the waters off Korea to fuel task forces. Several times she entered the heavily mined waters of Wonsan Harbor to fuel the ships carrying out the lengthy blockade and bombardment of that key port.

Returning to the west coast, she gave service as a training tanker until her second Korean tour, from 1 August to 10 December 1951. During this time she spent a month at Taiwan fueling the ships on duty in the Straits, and made three voyages to Korean waters from Sasebo. Overhaul and training on the west coast preceded her third Korean war deployment from 9 April to 5 January 1953, during which her duty was similar to that of her second. Her next tour of duty in the Far East was completed between 11 April and 27 November 1953.

Cimarron sailed to the Far East again between 14 June 1954 and 8 February 1955, during which she served as flagship of the support group for Operation "Passage to Freedom," the evacuation of refugees from Communist North Vietnam. Her pattern of operations from that time into 1963 has included support of the guardian 7th Fleet in its Far Eastern operations through deployments in 1955, 1956-57, 1957-58, 1958-59, 1959, and 1960. As of 1963, she had the longest continuous commissioned service of any active ship in the United States Navy, belying her age as she continued to provide her essential support with skill and efficiency.

Cimarron received 10 battle stars for World War II service, and four for the Korean war.