The Guy company did not have sufficient production capacity to produce sufficient Guy armoured cars as well as other vehicles, so shortly after war broke out the Rootes Group were approached to produce an armoured car – at the time the terminology "Tank, Light (Wheeled)" was used by the Army.

Karrier moved the KT4 engine to the rear and fitted welded bodies and turrets provided by Guy.

As it had been based on proven elements, trials of prototypes passed without serious issues and an order for 500 was placed in 1940 and the first deliveries made in 1941.

[3] The Karrier name was dropped to avoid confusion with the British Universal Carrier tracked vehicle[4] and the vehicles were designated "Armoured Car, Humber Mk 1" using the name of Humber Limited (another member of the Rootes Group) though production was by Karrier at the Luton works of Commer (another Rootes company).

[5] For forward vision the driver had a flap in the front of the "cab" (which became part of the glacis from the Mark II onwards).

A British Indian Army armoured car regiment, partly equipped with Humbers, served in the reconquest of Burma.

After the Second World War, the Humber was employed by Egypt in 1948–49 as well as by Burma, Ceylon, Cyprus, Denmark, India, Mexico and the Netherlands.

[8] The Humbers and Daimlers of the Indian Army formed the mounts of the President's Bodyguard and were deployed in the defense of Chushul at heights above 14,000 ft during the 1962 Indo-China War.