[2] On 7 December 1941, 30 minutes after receiving news of the Attack on Pearl Harbor, Hammann and the other ships in the North Atlantic Patrol left Reykjavík and steamed for the United States.

On 22 January 1942, she arrived in Pearl Harbor and joined Vice Admiral Frank Jack Fletcher's Task Force 17 for action in the South Pacific.

While screening the carriers during the air raids of 4 May, Hammann was directed to rescue two fighter pilots downed on Guadalcanal, some 40 miles (64 km) to the north.

Lexington, which had taken two devastating torpedo hits to port, was first thought to be under control, but a large internal explosion shortly before 13:00, followed later by others, sealed her fate.

Under urgent orders from Admiral Chester Nimitz to meet a new threat, Hammann moved to Pearl Harbor with the Task Force, arriving on 27 May.

During the air battle on 4 June, Hammann screened the carrier Yorktown, helping to shoot down many of the attacking aircraft.

Hammann picked up survivors in the water, including Yorktown's commanding officer, Captain Elliott Buckmaster, and transferred them to the larger ships.

The salvage party was making progress when the protective destroyer screen was penetrated by Japanese submarine I-168 after noon on 6 June.

Ensign Theodore E. Krepski was awarded the Navy Cross for his actions rescuing sailors abandoning Lexington at Coral Sea.