121 Heavy Anti-Aircraft Regiment RA (TA) was formed at Leigh on Sea in January 1941. The Regiment served in Anti-Aircraft Command until transferring to 2 Army in March 1944. It went to France in June 1944. 121 Heavy Anti-Aircraft Regiment RA (TA) was disbanded in March 1946.
Illustrations of the formation badges under which a unit served or supported are given. This is no guarantee that the unit used this formations badge.
| Jan 41 | Formed | Leigh on Sea, Essex, UK | |
| Jan 41 |  |
6 Anti-Aircraft Division, 37 Anti-Aircraft Brigade | South East England |
| May 41 | |
6 Anti-Aircraft Division, 29 Anti-Aircraft Brigade | South East England |
| Feb 42 | |
6 Anti-Aircraft Division, 6 Anti-Aircraft Brigade | South East England |
| Apr 43 | |
6 Anti-Aircraft Division, 37 Anti-Aircraft Brigade | South East England |
| Mar 44 |  |
2 Army | UK |
| Aug 44 | |
2 Army, 105 Anti-Aircraft Brigade | Normandy, France |
| Dec 44 | |
2 Army, 105 Anti-Aircraft Brigade | Scheldt, Belgium |
Batteries
| Feb 41 | 385, 387, 388 |
| May 42 | 385, 387, 388, 413 |
| Apr 43 | 385, 387, 388 |
Gun Sites Occupied
387 Battery
| Dates | Gun Site | Gun Defended Area |
| 1942 | H4 Shetley Street | Harwich |