During early 1941 the British Purchasing Commission in Washington asked the Americans if the M7 Priest could be altered to allow it to carry the British 25 pdr (87.6 mm/3.45 in) gun-howitzer. While the British appreciated the amenities of the M7 Priest, it had the major disadvantage of mounting a 105 mm (4.13 1n) howitzer that was not a standard British weapon calibre at that time. The Americans accordingly produced the M7 with the 25 pdr and named it the TB51 but at the same time announced that there was no way that they could produce it in quantity as they had their production hands full already. The British accordingly looked around and noted that the Canadians had set up a production line for the Ram tanks, a type that was soon to be replaced by the American M3 and M4. The Ram was accordingly altered to accommodate the 25 pdr, and thus was born the Sexton.

The Sexton mounted the British 25 pdr gun and was a well liked and reliable vehicle that served on for many years after World War II with many armies.

The Sexton used the overall layout of the M7 Priest, but many changes were introduced to suit British requirements, These included the movement of the driver's position to the right-hand side. The Sexton lacked the pronounced 'pulpit' of the M7, but the fighting compartment was left open with only a canvas cover to provide weather protection for the crew. The Sexton had a crew of six, and much of the interior was taken up with lockers for ammunition and some of the crew's personal kit; more stowage was provided in boxes at the rear. Maximum armour thickness was 32 mm. (1.25 in).

The 25 pdr gun howitzer was carried in a special cradle produced by the Canadians specifically for the Sexton. This allowed a traverse of 25' left and 40' right, which was very useful for the anti tank role (18 AP rounds) but in the event the Sexton had little need of this facility. Instead it was used almost exclusively as a field artillery weapon (87HE and smoke rounds) supporting the armoured divisions in North West Europe from 1944 onwards. There were several variations, all of them incorporating the production changes progressively introduced on the lines of the Montreal Locomotive Works at Sonar. Production continued there until late 1945, by which time 2,150 Sextons had been manufactured.

The Sexton was a well liked and reliable gun and weapon combination that proved so successful that many are still in use in odd corners of the world to this day. The British army used the type until the late 1950s, and one is preserved as a museum piece at the Royal School of Artillery at Larkhill in Wiltshire.

There were a few in service variants of the Sexton, some being converted to 'swim' for possible use on D Day, but none appear to have been used in this role on the day. A more common conversion was the replacement of the gun howitzer by extra map tables and radios in the Sexton Gun Observation Post command vehicle: there was usually one of these to a battery. In post war years some Sextons were handed over to nations such as Italy who preferred the 105 mm (4.13 in) howitzer; in this instance the 25 pounders were replaced with German 105 mm howitzers.