121 Heavy Anti-Aircraft Regiment RA(TA) was formed at Leigh on Sea in January 1941. The Regiment was disbanded in March 1946.
| Jan 41 | Formed | Leigh on Sea, Essex, UK |
| Jan 41 | 37 Anti-Aircraft Brigade, 6 Anti-Aircraft Division | South East England |
| May 41 | 29 Anti-Aircraft Brigade, 6 Anti-Aircraft Division | South East England |
| Feb 42 | 6 Anti-Aircraft Brigade, 6 Anti-Aircraft Division | South East England |
| Apr 43 | 37 Anti-Aircraft Brigade, 6 Anti-Aircraft Division | South East England |
| Mar 44 | 2 Army | UK |
| Aug 44 | 105 Anti-Aircraft Brigade, 2 Army | Normandy, France |
| Dec 44 | 105 Anti-Aircraft Brigade, 2 Army | Scheldt, Belgium |
Batteries
| Feb 41 | 385, 387, 388 |
| May 42 | 385, 387, 388, 413 |
| Apr 43 | 385, 387, 388 |
