Established by the members of the Ankh-Morpork City Watch during the short and disorderly reign of Fred Colon as Watch Captain, so to negotiate for the return of those positions and benefits that had been quickly cut by Acting-Captain Colon. Like all the other legalised guilds, the Watchmen's Guild was properly registered with the Patrician's Palace. Corporal "Nobby" Nobbs was elected to serve as the Guild's first president, and this was probably the highest leadership position he has ever achieved, since he had taken care to stay at the rank of corporal.
The Guild went on strike but never had a chance to actually negotiate with Acting-Captain Colon, who had barricaded himself inside the Watch headquarters at Pseudopolis Yard where he remained until Captain Carrot came back and gained entry to the building by force. During the strike the Watchmen discussed strategies and demands in their favorite pub The Bucket, a process that, while not very productive, but did serve to keep the striking-Watchmen relatively happy.
It is unclear whether the Watchmen's Guild continued after Mr. Vimes and Captain Carrot returned to Ankh-Morpork, called back those Watchmen who had been sacked, and restored Colon to his old comfortable rank of sergeant. Given the Patrician Lord Vetinari's habit of not breaking something that causes no harm, it is likely that this Guild nominally stays in existence, but is no-longer very active.