Promoted to Lieutenant (junior grade) in 1915, he was Commanding Officer of the submarine USS L-4 between 1916 and 1918, receiving the Navy Cross for "distinguished service" during World War I combat operations against German U-boats.

Following shakedown out of Bermuda, Lewis Hancock in company with Langley (CVL-27) sailed from New York 6 December for the Pacific; arrived Pearl Harbor on Christmas Day 1943; and joined Vice Admiral Marc A. Mitscher’s Fast Carrier Task Force (then 5th Fleet's TF 58, later 3rd Fleet's TF 38), a mighty naval weapon organized to neutralize Japanese airpower and forward bases in advance of leapfrogging American amphibious operations.

Assigned the task of neutralizing enemy airpower on Kwajalein Atoll, the flattops in Lewis Hancock’s group smashed the airdrome at Roi on the 29th, destroying every Japanese plane.

The destroyer accompanied the task force on the first strike against Truk, the major Japanese naval base in the Central Pacific, 16 and 17 February.

The Japanese attempted to counter the American thrust, into the Marianas by striking at the invading task force with their full naval strength.

The U.S. carriers, guarded by Lewis Hancock, smashed the enemy fleet in the Battle of the Philippine Sea 19 and 20 June, and thus saved the forces which were conquering the Marianas.

These airstrikes helped to neutralize Japan’s airpower and soften up her defenses for General Douglas MacArthur’s long-awaited return to the Philippines.

The U.S. troops landed on the beaches of Leyte, and Japan struck back with her full fleet in effort to stem the American advance.

In the ensuing Battle of Leyte Gulf, while acting as a picket ship, Lewis Hancock assisted in sinking an enemy destroyer.

Joining the 5th Fleet in February 1945, she participated in a series of raids against the Japanese home islands striking Tokyo on the 16th and 25th and the Kobe-Osaka area 19 March.

On Armed Forces Day, 19 May 1951, she recommissioned at the Naval Station, Long Beach, Calif. On 11 October she departed San Diego for the east coast and arrived Newport, R.I. on the 27th for modernization.

This Far Eastern deployment ended late in January 1953 when she departed Tokyo Bay for Newport via Southeast Asia, the Middle East, the Suez Canal and the Mediterranean.

Brought out of mothballs and modernized, Lewis Hancock was transferred to the government of Brazil on 1 August 1967, and was commissioned on the same day in the Brazilian Navy as CT Piaui (D 31).