King was born in Glasgow. She lived with Thomas Pearson, whom she met when she was pregnant out of wedlock and in poverty.[1][2] Pearson was of the middle-class, and had left his wife and children.[2] He was an alcoholic.[1][2] She had her own children, including a son named Thomas,[2] as well as adopted ones.
In October 1888, a group of people in Stockbridge found the body of an infant wrapped in oilskin in the street.[1][2] King and Pearson were suspected by their neighbors, who noted that three of their adopted children had disappeared.[3] When their house was raided, a second dead infant, a baby girl named Violet,[3] was found in the house's coal closet.[1] King claimed she was guilty and that Pearson had not known about either of the children.[1][2] She was arrested for the murder of three children, all of whom were the children of domestic servants or factory girls who had been adopted by King through newspaper ads.[1][2] Authorities at the time suspected there may have been more murdered children.[3]
Shortly after her arrest, King recanted her confession, claiming the killings were Pearson's idea, although this was not accepted by the authorities.[1] Her Catholic confessor asked the Secretary of State to reconsider, although again this was not accepted.[1][2][3] According to that confession, Pearson had killed one of the children himself, and had directed King to do so in the other two cases.[2][3]
King was kept in Calton Jail while awaiting trial.[3] She made several suicide attempts during this time.[3]
King's trial before the High Court in Edinburgh began in February 1889.[2] At her trial, Joseph Bell provided some medical expertise.[1] Pearson was granted immunity under the agreement that he testified against King.[1] King's mental health was assessed several times during the trial, but she ultimately deemed fit to be sentenced.[1] During the trial, the press presented King as a "fallen woman" who was irredeemably evil.[1][2]
The jury deliberated for four minutes.[3] King was found guilty and hanged in Edinburgh on 11 March 1889.[1][4] She was buried near what is now St. Andrew's House.[1][2] Pearson died in Glasgow in 1890.[2]