First Marine Amphibious Corps Headquarters
Established 1 Oct. 1942. Action on Guadalcanal, Russell Islands, New Georgia,  Bougainville, and Emirau. All 1st MAC assets were assigned to the III Amphibious Corps 15 April 1944.

First Marine Amphibious Corps Defense Battalion
Based on one of the Marines basic missions: "To assist the Fleet in the seizure and initial defense of such advanced bases as the fleet may require in its war operations." The 1st through 7th Defense Battalions were assigned through out the American sphere of influence with two of particular interest, The 5th was on Iceland and another detachment on Wake. A total of 20 Def. Bns. were eventually raised, but not all served w/ the 1st MAC. Starting in 1944 all Def. Bns. were redesignated as antiaircraft units.

First Marine Amphibious Corps Aviation Engineer
Composed of the 1st and 2nd Aviation Engineer Battalions. The first was attached to 1st Marine Aircraft Wing. The 2nd was deployed to Guadalcanal in 1943 and worked on Koli 1 & 2 airfields, bomb dump and dock areas. Transferred to III Amphib Corps in 1944.

First Marine Amphibious Corps Service & Supply
Forces included Supply units, 1st Corps Signal, Motor Transport and Medical Battalions. Elements moved forward with the Marine advances across the pacific and ensured support of the combat effort.