The Sexton was the most widely produced British Commonwealth self-propelled gun during World War II, born out of the failure of the Bishop to provide a reliable platform for the QF 25-pdr gun-howitzer. The poor performance of the Bishop resulted in the British Army adopting large numbers of the US-designed M7 Priest which despite its effectiveness proved to be a logistical challenge given that its 105-mm ammunition was not used by the British Army. Given the impossibility of the US producing a vehicle similar to the Priest with the 25-pdr, it was decided to offer the task to the Canadian Army Engineering Design Branch which chose the Ram tank (a Canadian-built version of the M4) as the chassis. The resulting vehicle somewhat resembled a more sharply angled version of the Priest, with a similar open-top gun compartment albeit without the prominent gun ring that gave its predecessor its name (there was no defensive armament on the Sexton). Despite the increased crew vulnerability, it also avoided the high silouhette of the Bishop. The Sexton was also faster and did not suffer the gun elevation issues of its predecessor allowing it to take full advantage of the 25-pdr's range. Sexton's were first employed by Canadian forces during the Italian campaign and both British and Canadian forces after D-Day eventually going on to replace the Priest. The Sexton went to serve for over a decade post-war, being retired by the British Army only until 1957, with India and Portugal also adopting it.

An early attempt to fit a 25-pdr gun on an M7 resulted in the US-built T51 but was plagued with difficulties and abandoned. The first Canadian prototype was built on 23 June 1942 and entered service as the Sexton Mk. I, based on the Ram chassis. Most were employed by the Canadian Army. The Sexton Mk. II was based on the Grizzly (Canadian-built M4A1) and accounted for the bulk of production, with the main user being the British Army. A battery fire control vehicle with the gun removed in place of an additional wireless radio was known as the Sexton GPO. A very similar post-war derivative of the Sexton was the Australian-built Yeramba which used the M3A5 chassis.