Mary Elizabeth Cassidy, known later as Mary Elizabeth Wilson, was born on June 11, 1889, in Catchgate, Stanley, County Durham, England. She was christened on June 30, 1889, at Our Blessed Lady and St. Joseph Church in Brooms, County Durham. Mary grew up in a time when life was often harsh in industrial England.

She married her first husband, John Knowles, around November 1914. They made their home in Windy Nook, Gateshead. Over the years, her personal life became complicated. Her lover, John Russell, eventually moved in with them. John's death in 1955 marked a significant change in her life. Just five months later, Mary married Russell. Unfortunately, he also died shortly after their marriage, in 1956 or early 1957. Both men’s deaths were declared to be due to natural causes by the attending physician.

Mary Elizabeth Wilson married her first husband, John Knowles, around November 1914. He died in 1955, and five months later, she married her lover John Russell. He died in 1956 or early 1957. Both men had their deaths ruled as natural causes. Wilson inherited money from both marriages.

In June 1957, Wilson married her third husband, Oliver Leonard. He died just 12 days after their wedding, leaving her £50. Shortly after, she married a fourth husband, Ernest Wilson. He died within a year, and she did not attend his funeral. By this time, she attracted local gossip. People noticed how often her husbands died and remarked on her cheerful attitude about it. She joked about leftover food being fresh for the next funeral and asked the local undertaker for discounts for giving him so much business.

The police started to investigate. They exhumed the bodies of her last two husbands and found high levels of phosphorus. Wilson's defense claimed this came from their medication. Eventually, she was convicted of murdering two of her four husbands with beetle poison. The remains of her first two husbands were also examined and showed similar causes of death. However, there was no second trial.

Mary Elizabeth Wilson was sentenced to death in 1958, but due to her age, her sentence was changed to life imprisonment. She died of natural causes on December 5, 1962, while in HM Holloway Prison.