Following her shakedown cruise, Tappahannock reported for duty with Commander, Service Force, Atlantic (ComServLant), on 13 August, and soon got underway south for Panama.

In the Canal Zone, the oiler took on board 300 tons of stores for Motor Torpedo Boat Squadron (MTBRon) 3, as well as two PT boats, and left Balboa on 29 August, bound for the South Pacific Tappahannock unloaded her cargo at Nouméa, New Caledonia, on 18 September; and then visited Auckland, New Zealand; Pago Pago, Samoa; and Nadi, Fiji, before becoming station tanker at Nouméa.

By early spring, Tappahannock was in the Guadalcanal area, as American forces sought to consolidate their hold on the bitterly contested Solomons.

On 1 April, a Grumman J2F Duck seaplane, attached to Enterprise (CV-6), crashed into the oiler's mainmast and damaged her radar tower and antenna.

The first Japanese dive bomber came in from off the port quarter — as the ship was swinging to starboard to evade the attack — and dropped her bomb abaft the bridge.

While Tappahannock's "black gang" below valiantly fought to get underway again, a second attacker came in from off the oiler's starboard bow, on a course diagonally across the ship and soon entered the concentrated gunfire of one three-inch and three 20-millimeter guns.

However, the "Val" stubbornly remained airborne and escaped the hail of gunfire; but no sooner had the second aircraft been driven off when three of the oiler's 20-millimeter guns jammed.

Apparently — and fortunately for Tappahannock — the momentary power-loss had been a good thing since the Japanese pilot misjudged his target's speed and ended up dropping his bomb clear of the ship — the near miss drenching the forecastle anti-aircraft gunners with water from the splash.

His bomb detonated just alongside, causing more severe vibrations than the first near miss and leading to fears that, this time, the ship had suffered serious harm.

The oiler arrived at Espiritu Santo soon thereafter, and repair crews from Vestal (AR-4) remedied her topside damage and patched her hull below the waterline.

After her repairs had been completed, Tappahannock resumed active service as American forces continued to strengthen their hold on the Solomon Islands.

The oiler continued supporting Allied operations in the South Pacific until late August when she headed home to the west coast.

She then conducted two voyages from San Pedro to Pearl Harbor before heading west to support the American invasion of the Gilbert Islands in November and December.

For the remainder of the war, Tappahannock conducted vital fueling duties for the Fleet as it pounded westward and northward against the Japanese empire.

She supported the occupations of Kwajalein, Majuro, Guam, Tinian, the Southern Palaus, Luzon, Okinawa and aided the fast carriers in their raids on the Bonins, Philippines, and Formosa.

In the course of these operations, she conducted underway replenishments with the task forces and served as station tanker in newly occupied lagoons and harbors.

The oiler remained, as station tanker, at the one-time German colonial showplace city until 26 April 1946, when she got underway for the Near East to load a cargo of oil.

Tappahannock returned to the China Coast in the late spring, making port at Hong Kong, British Crown Colony, on 7 June.

Departing Hong Kong on 6 August, she sailed again for the Near East, for Ras Tanura and Colombo, to load another cargo of petroleum products for delivery to the fleet serving on occupation duty in Japanese and Chinese coastal waters.

The oiler returned to the Mediterranean soon thereafter, transited that body of water and the Suez Canal, and arrived at the Persian Gulf port of Bahrain on 5 April.

Early in 1949, she again headed west for the Far East and thence proceeded across the Mediterranean to operate for a time in northern European waters, calling at Bremerhaven, Germany, and Cherbourg, France, before returning to Norfolk on 4 June.

However, Tappahannock's sojourn in reserve was brief; for, before mid-year, communist aggression in Korea triggered a build-up of the Navy's fleets in both oceans to respond to the threat.

During the course of these operations, she ranged from the Canal Zone to the Firth of Forth; from Argentia, Newfoundland to Port Arthur, Texas; and in the North Atlantic to the Caribbean and the eastern seaboard of the United States.

Tappahannock supported the 7th Fleet's "Yankee Team" and "Market Time" operations interdicting communist seaborne supply lanes.

On 13 June 1967, during an underway replenishment in the South China Sea, USS Repose (AH-16) suffered a rudder failure and collided with Tappahannock.

The oiler, by now one of the oldest active-duty ships on the Navy list, was clearly aging; and replacements for many of her worn-out parts were hard to find or unavailable.