La Voisin was initially a fortune-teller, but soon extended her services to fulfilling the fortunes of her clients. Utilizing both alchemy and black masses, she created a subtle and effective poison which had no smell and left no trace. She used the poison by employing various trick jewelry with hidden compartments. Holding little affection for her husband or sons, La Voisin was said to have attempted or plotted to poison them.
However, her daughter, Marguerite Monvoisin, appears to have assisted in her plots, most famously a series of scandals known as the Affair of the Poisons. According to information gathered by the authorities in charge of uncovering La Voisin's conspiracy, La Voisin and Madame de Montespan, the former official mistress of King Louis XIV, had plotted against the king. La Voisin was convicted of conspiracy, and was condemned to death and burned at the stake at the Place de Grève on 22 Februrary 1680. Nonetheless, Marguerite continued the family traditions.
During the French Revolution, La Voisin's descendant, Amélie Monvoisin, poisoned her sister, Valérie, with the same methods as La Voisin, hoping to inherit the fortune of the Monvoisin family. However, the Assassin Arno Dorian uncovered the murder, and Amélie was imprisoned.
- La Voisin is French for 'the Neighbor'.