In 1868, Mary worked as an assistant at a small pharmacy in the City of London, working under Thomas Reese. At some point, she got into an affair with industrialist John Ashton, who was known as the most hated man in London. When she learned about his true character, and wanted to end their affair, Ashton refused to allow it.
Mary then resorted to murder, poisoning an apple and making sure a merchant would sell it to Ashton when he passed by on his daily routine. Unbeknownst to her, several other also attempted to murder Ashton that day, resulting in someone else being Ashton's actual murderer, since the man expired from a stab wound before Mary's poison had run its course.
Mary was questioned by the Assassins Jacob and Evie Frye. When the Frye twins finished their investigation, however, they only apprehended the actual murderer, letting the would-be murderers go unpunished.