121 (West Riding) Medium Regiment RA (TA) was formed by the conversion of 121 Field Regiment in January 1944. It went into suspended animation in April 1946. 121 (West Riding) Medium Regiment RA (TA) was reconstituted as 321 (West Riding) Heavy Anti-Aircraft Regiment RA (TA) in January 1947.
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 44 | Formed from 121 Field Regiment | Felixstowe, Suffolk, UK | |
| Jan 44 |  |
2 Army, 5 Army Group RA | Felixstowe, Suffolk, UK |
| May 44 | |
2 Army, 5 Army Group RA | Clacton, Essex, UK |
| Jun 44 | |
2 Army, 5 Army Group RA | Rucqueville, France |
| Jun 44 | |
2 Army, 5 Army Group RA | Audrieu |
| Jul 44 | |
2 Army, 5 Army Group RA | Caumont, France |
| Aug 44 | |
2 Army, 5 Army Group RA | Le Parquet, France |
| Aug 44 | |
2 Army, 5 Army Group RA | Vernon, France |
| Oct 44 | |
2 Army, 5 Army Group RA | Nijmegen, Holland |
| Oct 44 | |
2 Army, 5 Army Group RA | Antwerp, Holland |
| Nov 44 | |
2 Army, 5 Army Group RA | Rosendaal, Holland |
| Nov 44 | |
2 Army, 5 Army Group RA | Schinveldt, Holland |
| Nov 44 | |
2 Army, 5 Army Group RA | Tevern, Germany |
| Dec 44 | |
2 Army, 5 Army Group RA | Liége, Belgium |
| Dec 44 | |
2 Army, 5 Army Group RA | Mont Gauthier, Belgium |
| Jan 45 | |
2 Army, 5 Army Group RA | Baillonville, Belgium |
| Jan 45 | |
2 Army, 5 Army Group RA | Monville, Luxembourg |
| Jan 45 | |
2 Army, 5 Army Group RA | Cheoux, Luxembourg |
| Feb 45 | |
2 Army, 5 Army Group RA | Mook, Holland |
| Feb 45 | |
2 Army, 5 Army Group RA | Reichswald |
| Feb 45 | |
2 Army, 5 Army Group RA | Hau, Germany |
| Feb 45 | |
2 Army, 5 Army Group RA | Goch, Germany |
| Mar 45 | |
2 Army, 5 Army Group RA | Bönninghardt, Germany |
| Mar 45 | |
2 Army, 5 Army Group RA | Appeldorn, Holland |
| Mar 45 | |
2 Army, 5 Army Group RA | Bienen, Germany |
| Apr 45 | |
2 Army, 5 Army Group RA | Nordlohne, Germany |
| Apr 45 | |
2 Army, 5 Army Group RA | Bremen, Germany |
Batteries
| Jan 44 | 275, 276 |
Equipment
| Jun 44 | 5.5 inch Guns, Matador Tractors |