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 5–16 September she put to sea with a transport convoy bound for Iceland, and voyaged north again from 12 October to 5 November to refuel ships at Placentia Bay. On 15 November 1941, she joined a convoy at Trinidad bound for Singapore with reinforcements, but was detached from the convoy on 9 December at Cape Town, South Africa. Returning to Trinidad on 31 December, she operated from Brazilian ports t