The USAF operated EC-121s between 1954 and 1978, with 3 wings at maximum employment and 3 independent squadrons as operations wound down.
Until the Vietnam War, their primary mission was to provide complementary early warning radar coverage to the Pacific and Atlantic barriers by flying orbits 300 miles offshore from the continental United States in what was termed "Contiguous Barriers".
The coverage orbits overlapped those of land-based early warning radars.

Initial deployment of EC-121Cs began with 551st Airborne Early Warning and Control Wing, based at Otis Air Force Base, Massachusetts.
Operational on 21 December 1954, 551st AEWCW subsequently upgraded to EC-121D and later EC-121H Warning Stars.

Its Pacific counterpart was 552d Airborne Early Warning and Control Wing (AEWCW), based at McClellan Air Force Base, California, becoming operational on 1 July 1955.
After the Cuban Missile Crisis in 1962, 552nd AEWCW also had administrative control of the 966th AEWCS, based at McCoy Air Force Base, Florida.
The 966th had a dual mission: monitoring activity in Cuban airspace and flying Gold Digger missions (continuous tracks of U-2 surveillance missions).

The 3rd air wing to operate EC-121s was 553rd Reconnaissance Wing, a Vietnam war organization activated in 1967 and based in Thailand until inactivation in 1970 (See BatCat below).
In 1966 Lockheed modified 30 ex-USN Super Constellations (2 EC-121P/WV-3 and 28 EC-121K/WV-2) aircraft to EC-121Rs for the specialized recon mission flown by the 553rd.
Aircraft were delivered during the course of 1967.
553rd RW flew over land and off the coast of Vietnam, over Laos and Cambodia, monitoring and retransmitting low-power signals.
Usually they orbited the Ho Chi Minh Trail in 8-hour shifts.
As it was expensive to operate and it exposed a large crew to enemy fire it was replaced in December 1970 by the much smaller (and eventually unmanned drone) QU-22 Pave Eagle sensor monitor.

551st AEWCW inactivated in 1969, while the 552d was reduced by a squadron in 1971.
In July 1974, USAF redesignated 552d Airborne Early Warning and Control Wing as 552d Airborne Warning and Control Group when it was downsized to a single squadron.
 It inactivated in April 1976.

As the USAF prepared for deploying the E-3 Sentry in the later 1970s, it phased out EC-121 operations by the end of 1975.
All remaining EC-121s transferred to Air Force Reserve, which formed the 79th AEWCS at Homestead Air Force Base, Florida in early 1976.
The active duty force continued  providing personnel to operate the EC-121s on a 24-hour basis, assigning Detachment 1, 20th Air Defence Squadron to Homestead AFB as associate active duty crews to fly the AFRES-owned aircraft.
Besides monitoring Cuban waters these last Warning Stars also operated from NAS Keflavik, Iceland.
Final EC-121 ops ended in September 1978.
Detachment 1 dissolved and 79th AEWCS was redesignated a fighter squadron on 1 October 1978.

In 1967, 5 EC-121S became operational with the 193rd Tactical Electronic Warfare Squadron of the Pennsylvania Air National Guard, responsible for psyops missions under project Coronet Solo.
From July 1970 to January 1971 they rotated on 30- to 90-day temporary duty deployments to Korat Royal Thai Air Force Base, Thailand, under the name Commando Buzz.
The USAF operated EC-121s between 1954 and 1978, with 3 wings at maximum employment and 3 independent squadrons as operations wound down. Until the Vietnam War, their primary mission was to provide complementary early warning radar coverage to the Pacific and Atlantic barriers by flying orbits 300 miles offshore from the continental United States in what was termed "Contiguous Barriers". The coverage orbits overlapped those of land-based early warning radars.
Initial deployment of EC-121Cs began with 551st Airborne Early Warning and Control Wing, based at Otis Air Force Base, Massachusetts. Operational on 21 December 1954, 551st AEWCW subsequently upgraded to EC-121D and later EC-121H Warning Stars. 
Its Pacific counterpart was 552d Airborne Early Warning and Control Wing (AEWCW), based at McClellan Air Force Base, California, becoming operational on 1 July 1955. After the Cuban Missile Crisis in 1962, 552nd AEWCW also had administrative control of the 966th AEWCS, based at McCoy Air Force Base, Florida. The 966th had a dual mission: monitoring activity in Cuban airspace and flying Gold Digger missions (continuous tracks of U-2 surveillance missions).
The 3rd air wing to operate EC-121s was 553rd Reconnaissance Wing, a Vietnam war organization activated in 1967 and based in Thailand until inactivation in 1970 (See BatCat below). In 1966 Lockheed modified 30 ex-USN Super Constellations (2 EC-121P/WV-3 and 28 EC-121K/WV-2) aircraft to EC-121Rs for the specialized recon mission flown by the 553rd. Aircraft were delivered during the course of 1967. 553rd RW flew over land and off the coast of Vietnam, over Laos and Cambodia, monitoring and retransmitting low-power signals. Usually they orbited the Ho Chi Minh Trail in 8-hour shifts. As it was expensive to operate and it exposed a large crew to enemy fire it was replaced in December 1970 by the much smaller (and eventually unmanned drone) QU-22 Pave Eagle sensor monitor.
551st AEWCW inactivated in 1969, while the 552d was reduced by a squadron in 1971. In July 1974, USAF redesignated 552d Airborne Early Warning and Control Wing as 552d Airborne Warning and Control Group when it was downsized to a single squadron.  It inactivated in April 1976. 
As the USAF prepared for deploying the E-3 Sentry in the later 1970s, it phased out EC-121 operations by the end of 1975. All remaining EC-121s transferred to Air Force Reserve, which formed the 79th AEWCS at Homestead Air Force Base, Florida in early 1976. The active duty force continued  providing personnel to operate the EC-121s on a 24-hour basis, assigning Detachment 1, 20th Air Defence Squadron to Homestead AFB as associate active duty crews to fly the AFRES-owned aircraft. Besides monitoring Cuban waters these last Warning Stars also operated from NAS Keflavik, Iceland. Final EC-121 ops ended in September 1978. Detachment 1 dissolved and 79th AEWCS was redesignated a fighter squadron on 1 October 1978.
In 1967, 5 EC-121S became operational with the 193rd Tactical Electronic Warfare Squadron of the Pennsylvania Air National Guard, responsible for psyops missions under project Coronet Solo. From July 1970 to January 1971 they rotated on 30- to 90-day temporary duty deployments to Korat Royal Thai Air Force Base, Thailand, under the name Commando Buzz.
Lockheed EC-121 Warning Star