138 Heavy Anti-Aircraft Regiment RA (TA) was formed at Llanion Barracks, Pembroke Dock in November 1941. The Regiment served with Anti-Aircraft Command for its entire service. 138 Heavy Anti-Aircraft Regiment RA (TA) was disbanded in October 1945 at Hartlip, Sittingbourne, Kent.
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.
| Nov 41 | Formed | Pembroke Dock, Dyfed, UK | |
| Nov 41 |  |
9 Anti-Aircraft Division, 61 Anti-Aircraft Brigade | South Wales |
| Aug 42 | |
9 Anti-Aircraft Division, 67 Anti-Aircraft Brigade | South Wales |
| Sep 43 |  |
3 Anti-Aircraft Group | UK |
| Mar 44 | |
2 Anti-Aircraft Group, 102 Anti-Aircraft Brigade | South East England |
| May 44 | |
6 Anti-Aircraft Group, 57 Anti-Aircraft Brigade | Portsmouth, Hampshire, UK |
| Aug 44 | |
2 Anti-Aircraft Group, 43 Anti-Aircraft Brigade | South East England |
| Sep 44 | |
2 Anti-Aircraft Group, 5 Anti-Aircraft Brigade | Southern England |
| Mar 45 | |
9 Anti-Aircraft Group, 28 Anti-Aircraft Brigade | East Anglia, UK |
Batteries
| Dec 41 | 419, 424, 438, 474 |
| Apr 42 | 419, 424, 438 |
| Jul 42 | 419, 424, 437, 438 |
| Feb 45 | 419, 424, 437 |