Commissioned Ensign on 12 June 1941, he was assigned to Naval Air Station, San Pedro, California, and later to the USS Arizona.

Based at Manus, and later Ulithi, Halloran continued this duty until on 29 November, helping to bring vital fuel to fleet units off Palau, Yap, and Luzon.

While moored at Ulithi on 20 November, she witnessed the torpedoing of oiler Mississinewa (AO-59) and in company with Rail (AM-26) searched the harbor fruitlessly for a suspected midget submarine.

Halloran was underway again on 22 January 1945, this time as a unit of the escort screen for the amphibious task force designated to capture Iwo Jima.

After repairs at Kerama Retto, Halloran patrolled off Ie Shima from 5 to 13 July, then assumed tactical command of an escort screen for LST's en route to the Philippines.