Marine Ground Control Intercept Squadron 21 (MGCIS-21) was commissioned on 15 May 1947 at Naval Air Station Squantum, Massachusetts.

They provided early warning and ground control intercept as part of the Marine Air Reserve Training Command until they were decommissioned on 30 November 1950.

The squadron was quickly repurposed and brought back on 1 August 1962 as Marine Air Traffic Control Unit 73 (MATCU-73).

MATCU-73 was responsible for providing air traffic control and precision approach radar services at NAS South Weymouth.

After the Gulf War and in light of the Base Realignment and Closure (BRA) military downturn, the unit was relocated to Westover Air Force Base and occupied duplexes for 9 years until a new Marine Reserve Center was built with the politic backing of both sides of the political aisle.