Bill Williamson is one of the gang members that John Marston is ordered to track down. According to speech between Bill and John, the two were very close, though John apparently looked down on Bill for being less intelligent than him. This is evidenced by the fact Bill says John 'always had a thing for fancy words', and other such things.
Bill was an ally of both Javier Escuella and Dutch van der Linde, alongside John Marston. However, during a bank robbery in 1908 that went horribly wrong, John was shot and left for dead. After this Dutch went into a semi-retired state, and prompted Williamson to form his own gang in New Austin. The gang use the remains of Fort Mercer for their activities, and are well locked down. After John goes to Bill and tries to reason with him and asks him to surrender, Williamson mocks him. Marston reaches for his revolver, and prompts Williamson's men to fire their rifles at him. Wounded, Marston's body is found by Bonnie MacFarlane and a fellow farmer who was running errands with her. He is taken to a doctors, and given medical aid. After helping others around the landscape of New Austin, John manages to organize a group of people who are prepared to help him assault Fort Mercer.
Upon assaulting the fort, one of the bandits surrenders to the attackers, and they are able to glean some information off of him. He tells them that Bill has fled several days before and that he has gone across the border into Mexico alongside Marston's old friend Javier Escuella. The two men are friendly with Colonel Allende, the tyrannical governor of Nuevo Paraiso. Marston must work under Allende in an attempt to convince the military leader to hand over the two outlaws that Marston is seeking. Allende attempts to betray Marston, who survives the attack, and is saved by Abraham Reyes. He then helps Reyes lead the attack against Allende's villa. After doing this, Allende and Williamson flee in an armored stagecoach. After immobilizing the coach, Allende exits the stagecoach and tries to plead for his freedom by handing over Williamson. Should you choose to shoot Allende first, Williamson will fake surrender before attempting to draw his weapon. If Williamson is shot first, Allende will be killed by Reyes. Regardless, it is not possible to capture Bill.