The unit was first activated during World War II, being formed on February 1, 1943, at Marine Corps Base Camp Lejeune, North Carolina, under the designation of "Headquarters Company, 3d Separate Battalion". Later during the year, the battalion was relocated to Marine Corps Base Camp Pendleton, California, and redesignated as the "3rd Battalion 24th Marine Regiment", and assigned to the 4th Marine Division. During 1944, 3/24 was deployed to the Marshall Islands and participated in the Battles of Kwajalein, Saipan, Tinian, Roi Namur, and Iwo Jima. The battalion commanders of the unit during this period were Lieutenant Colonels Austin R. Brunelli and Alexander A. Vandegrift, Jr.—son of Commandant of the Marine Corps Alexander Vandegrift—until he was severely wounded on Iwo Jima. The fighting on Iwo Jima was fierce and losses amongst the battalion were high. When the battalion reembarked after 35 days of fighting, one of its companies, Captain William T. Ketcham's Company I, which had landed on Iwo Jima the first day with 133 Marines in its three rifle platoons, had only nine of these men remaining. After this, during 1945 the battalion relocated to Maui, in Hawaii, before moving to Camp Pendleton, California. Following the end of the war, the battalion was deactivated on October 31, 1945.