Mary Clement, also known as Marie Clément, was born on February 16, 1863, in Harlange, a small village in Luxembourg. She was the second of five daughters born to Michel Clément and Margarite Deville. In 1871, when Mary was eight years old, her family decided to move to the United States. They settled in Dubuque, Iowa. Three years later, in 1874, the last of the five sisters, named Annie, was born.

Mary was often described as slender and pretty. However, she had a defect in her spinal column that limited her control over her legs and feet. This condition affected her mobility in a significant way. Despite her physical challenges, Mary managed to grow up in a bustling household with her sisters.

Beginning in 1880, there were a series of unexpected deaths in Mary Clement's family. Her little sister, Annie, died on August 1, 1880, at the age of six after suffering from convulsions. Her death was attributed to eating too much before going to bed. Over the next few years, Mary’s family continued to suffer. Her mother, Margarite, passed away on July 24, 1884, at the age of 54, and her younger sister, Lena, died shortly after on August 9, 1884, at age 13. Their father, Michel, died on March 28, 1885, at the age of 49. All these deaths were linked to convulsions or heart failure from an unknown illness.

After her father's death, Mary moved in with her sister Catherine and her family in Rose Hill, Illinois. During her stay, Mary cooked meals for her sister's family. After each meal, they experienced violent vomiting and spasms. Her brother-in-law, Michael Freres, began to grow suspicious after noticing that Mary never ate the soup she prepared. One day, Michael found a pack of gray powder in his backyard. After eating some of the soup, he discovered a similar substance in his plate and the plates of his family. Concerned, he took the powder and the plate samples for examination, which revealed that the powder contained arsenic.

Mary was arrested shortly after this discovery. While in jail, Mary initially claimed her innocence and made accusations against her sister, stating that it was for the sake of money. Later, she confessed to poisoning the Freres family and admitted to killing her mother, father, and sister as well. She said that she wanted to end her mother's suffering. During her trial, evidence of the poison was presented, and with Michael as the key witness, Mary was sentenced to one year in jail. A month after her sentencing, she wrote a letter that included her confessions regarding all the murders, which surprised those who believed she was innocent. She expressed satisfaction with the short penalties she received.

After her release from prison, Mary moved to Los Angeles and took up work as a domestic servant for a wealthy family. She rarely spoke of her past and was regarded as a pleasant elderly woman. Mary Clement died at the age of 81 and was buried in the Freres family plot at St. Henry's Church in Chicago.