Early in World War II, Mahan took part in raids on the Marshall and Gilbert Islands. In the Battle of the Santa Cruz Islands, Admirals Chester Nimitz and William Halsey commended the destroyer group (of which Mahan was a member) for a stellar effort in screening the aircraft carriers and against heavy odds. During the New Guinea campaign to take the northeast coast from the Japanese, Mahan was engaged in the amphibious landings at Salamaua, Lae, and Finschhafen. She participated in landings at Arawe and Borgen Bay (near Cape Gloucester), New Britain, and provided support for the troop landing at Los Negros Island in the Admiralty Islands.
