After service off the west coast and in Hawaiian waters, Sangamon shifted to the Atlantic Fleet in the spring of 1941, and through the Neutrality Patrol period, carried fuel from the gulf coast oil ports to bases on the east coast, in Canada, and in Iceland.
On 7 December 1941, when the U.S. entered World War II, she was at Naval Station Argentia, Newfoundland, offloading her liquid cargo.
Within the week, she started south again to renew her schedule on a tighter time frame.

In early 1942, she was designated for conversion to an auxiliary aircraft carrier.
On 11 February, she arrived in Hampton Roads.
Three days later, she was reclassified AVG-26; and on 25 February, she was decommissioned and conversion was begun at the Norfolk Navy Yard.

During the spring and summer, the need for auxiliary carriers, later called escort carriers, increased.
Work on Sangamon, three other Cimarron-class oilers, and 20 C-3 merchant hulls was continued and sped up.
In August, Sangamon — the first of her class of escort carriers – was ready.
Her conversion had added a flight deck 502 ft (153 m) long and 81 ft (25 m) wide, elevators, a hangar deck, an aircraft catapult, sonar gear, aircraft ordnance magazines, workshops, and stowage space for aviation spares.
Her accommodations had been enlarged to house her increased complement and embarked aviation personnel, and her armament had been changed to two 5"/51 caliber guns, eight 40 mm guns, and twelve 20 mm cannons to increase her anti-aircraft defense.
On 20 August, she was redesignated ACV-26; and five days later, she was recommissioned, with Captain C. W. Wieber in command.

Sangamon and her three sister T3 conversions were considered very successful escort-carrier designs, larger and more stable than the smaller C3-derived Bogue class; additionally, they retained substantial oil bunkerage, useful in refueling destroyers in company.
The late-war purpose-built Commencement Bay-class escort carriers were derived from the Sangamons.
After service off the west coast and in Hawaiian waters, Sangamon shifted to the Atlantic Fleet in the spring of 1941, and through the Neutrality Patrol period, carried fuel from the gulf coast oil ports to bases on the east coast, in Canada, and in Iceland. On 7 December 1941, when the U.S. entered World War II, she was at Naval Station Argentia, Newfoundland, offloading her liquid cargo. Within the week, she started south again to renew her schedule on a tighter time frame.
In early 1942, she was designated for conversion to an auxiliary aircraft carrier. On 11 February, she arrived in Hampton Roads. Three days later, she was reclassified AVG-26; and on 25 February, she was decommissioned and conversion was begun at the Norfolk Navy Yard.
During the spring and summer, the need for auxiliary carriers, later called escort carriers, increased. Work on Sangamon, three other Cimarron-class oilers, and 20 C-3 merchant hulls was continued and sped up. In August, Sangamon — the first of her class of escort carriers – was ready. Her conversion had added a flight deck 502 ft (153 m) long and 81 ft (25 m) wide, elevators, a hangar deck, an aircraft catapult, sonar gear, aircraft ordnance magazines, workshops, and stowage space for aviation spares. Her accommodations had been enlarged to house her increased complement and embarked aviation personnel, and her armament had been changed to two 5"/51 caliber guns, eight 40 mm guns, and twelve 20 mm cannons to increase her anti-aircraft defense. On 20 August, she was redesignated ACV-26; and five days later, she was recommissioned, with Captain C. W. Wieber in command.
Sangamon and her three sister T3 conversions were considered very successful escort-carrier designs, larger and more stable than the smaller C3-derived Bogue class; additionally, they retained substantial oil bunkerage, useful in refueling destroyers in company. The late-war purpose-built Commencement Bay-class escort carriers were derived from the Sangamons.
USS Sangamon (CVE-26)