At some point prior to , Mary-Beth got into trouble when she stole watches and wallets from several men. She was found by the Van der Linde gang while being chased by her pickpocketing victims, and was presumably rescued by them before joining the gang. Mary-Beth soon finds a lead on a lucrative train robbery. Later, she and the other female members of the camp can talk to Arthur about his recent events.
Not long after, Sean MacGuire is rescued from Blackwater and the gang have a party. Mary-Beth asks Arthur to dance with her, and the player can either accept or decline. If Arthur is drunk Mary-Beth will ignore him and will not ask Arthur to dance with her. Mary-Beth can assist Arthur and Sean in robbing a stagecoach.
She distracts the drivers to allow Arthur and Sean to attack it. At the end, she is captured but Arthur saves her by killing the guard who held her hostage. Afterwards, she, Arthur and Sean divide the money between themselves and return to camp.
Not long after setting up camp at Shady Belle , Arthur comes across Mary-Beth reading and goes to talk with her. After talking about their pasts over a cigarette, Susan Grimshaw interrupts them about the disappearance of Tilly Jackson. Not long afterwards, Mary-Beth screams upon witnessing the mutilated corpse of Kieran Duffy enter camp on horseback as the O'Driscolls attack the gang.
She later expresses her grief over his death. Arthur can tell Mary-Beth that he is terminally ill, diagnosed with Tuberculosis. Mary-Beth gives him her sincere condolences, but tries to cheer Arthur up by encouraging to make his last moments count. As the gang begins to fall apart due to Dutch's increasingly erratic behavior and decisions, Mary-Beth eventually leaves alongside Simon Pearson , Karen Jones and Uncle.
