MAG-21 suffered 17 casualties and all 21 of their aircraft were destroyed during the December 7, 1941 attack on Pearl Harbor.
 Squadrons from MAG-21 fought at the Battle of Wake Island, Battle of Midway and as part of the Cactus Air Force during the Battle of Guadalcanal but the group itself remained at MCAS Ewa until February 1943 when they departed for Banika in the Russell Islands.
 By June 1943, MAG-21's squadrons were fighting large scale operations against Japanese installations in the northern Solomon Islands.
 The group moved to Efate in November 1943 and remained there until June 1944.
 Their final destination during the war was on Guam where they were based from August 1944 until the end of the war.

The squadron returned to Marine Corps Air Station Miramar in March 1946 with follow on movement to Marine Corps Air Station Cherry Point, North Carolina.
 They were deactivated in April 1947.
MAG-21 suffered 17 casualties and all 21 of their aircraft were destroyed during the December 7, 1941 attack on Pearl Harbor.  Squadrons from MAG-21 fought at the Battle of Wake Island, Battle of Midway and as part of the Cactus Air Force during the Battle of Guadalcanal but the group itself remained at MCAS Ewa until February 1943 when they departed for Banika in the Russell Islands.  By June 1943, MAG-21's squadrons were fighting large scale operations against Japanese installations in the northern Solomon Islands.  The group moved to Efate in November 1943 and remained there until June 1944.  Their final destination during the war was on Guam where they were based from August 1944 until the end of the war.
The squadron returned to Marine Corps Air Station Miramar in March 1946 with follow on movement to Marine Corps Air Station Cherry Point, North Carolina.  They were deactivated in April 1947.
Marine Aviation Training Support Group 21