The War Office was having second thoughts about this idea just as the prototype appeared, for the Spanish Civil War had not shown light tanks to be as effective as everyone had hoped, and it was not until 1940, when tanks of all kinds were desperately short, that a production order was given. Production was slow, partly because of difficulties with the unusual suspension, and partly because the factory was bombed. Some of the first production models went to Russia, and others took part in the Madagascar operations; the remainder were put in reserve straight from the factory to be kept as airborne tanks. The Hamilcar glider was specifically designed to carry the Tetrarch, but only one squadron flew to Normandy in 1944. One Hamilcar broke up while crossing the English Channel as a result of flying into the slipstream of the towing plane, and its Tetrarch and crew plunged into the sea. A few more Tetrarchs were used in the Rhine crossing in May 1945, but after that they were never used again, although a dwindling number remained on strength until the gliders disappeared in 1950.

The hull was a light box, with vertical sides and within the track width (not built out over the tracks, that is). In front was a large sloping glacis plate marred by a central square box which covered the driver's head and shoulders. The entire front of this box could be swung open to allow the driver to see, but when it was closed down the driver had only a small vision block in the middle of it. The glacis plate was 0.63in (16mm) thick, as was the front of the turret, but the remaining armour was only 0.24in (6mm) thick and highly vulnerable. The turret was built up from plate, much like that of the Mk.VI light tank, and had a big square mantlet housing the 2pounder gun. The turret was reasonably roomy, but the view was poor and there was no cupola for the commander. He was in any case very busy since he acted as loader for the 2pounder. It may be no accident that on the few occasions when the Tetrarch was in action it was generally used as a static gun, where the commander would have been able to give all his attention to controlling his gunner. There was a smoke discharger on each side of the turret, and most Tetrarchs had a spare petrol tank in a frighteningly vulnerable position on the rear decking.

The Meadows engine drove to a gearbox which gave five forward speeds and drove through the rear road wheel. The two centre wheels could be moved out or in, to bow the track for steering gentle curves. The idea was derived from the Bren Carrier, but was greatly improved on the Tetrarch and worked very well. Track bowing was controlled by a steering wheel, but sharper turns required the driver to use two levers and apply brakes to the final drive-shafts.

By the time the Tetrarch was ready for service, the 2pounder was already well out-classed, and in an attempt to improve its performance a Littlejohn adaptor was fitted. This gave a higher muzzle velocity, but it could not be used with HE shell, and so was only a marginal improvement.

It seems extraordinary that with the known failings of the Tetrarch the War Office should have ordered a successor on the same pattern, but they did and 102 were built by 1944. This tank was the Mark VIII, or Harry Hopkins as it was felicitously named. Mechanically it was very similar to the Tetrarch, but the hull was simplified in outline and had a long flat top deck with a steeply sloping plate. This last was in one piece and 1.5in (38mm) thick. The turret was lower and had sloped sides, though the front plate was still flat, but a better mantlet was fitted. The armament was unchanged, and the weight went up to 19.040lb (8,636kg). This tank was never used in action (few were even issued for service use), and it could be described as a waste of factory effort at a time when such effort was badly needed for other things.

A derivation of the Harry Hopkins was the Alecto self-propelled gun, developed in response to a General Staff requirement of April 1942 for a light self-propelled gun for infantry support, using the 95mm howitzer. The Alecto used a totally redesigned hull from the Harry Hopkins, with the gun mounted low down in the front. The crew was four, the height reduced by nearly 9in (227mm) and the speed reduced to 30mph (48km/h). In order to keep the weight down as much as possible, the armour was only 0.4in (10mm) at its thickest, but it was all-welded, a new technique at that time. Had the Alecto gone into action it seems quite likely that it would not have survived for long, for all SP guns were the target for the heaviest retaliation that could be fired at them, and 10mm of armour would not have kept out any AP projectiles. As it happened, few were built and none saw action. After the war a very small number were tried as reconnaissance vehicles, with limited success.