She was born Catherine Deshayes around 1640 and married Antoine Monvoisin, a jeweller whose business failed. She supported the household as a fortune teller and midwife in the faubourg Saint-Denis, and built from that a large and profitable practice: palm reading and face reading for the fashionable, abortions for those who needed them, and, for clients who wanted a marriage ended or an inheritance brought forward, powders.
Her clientele reached the court. Through her the Marquise de Montespan, the king's official mistress, was said to have bought preparations to hold Louis XIV's affection and to injure a rival. The investigation that followed her arrest on March 12, 1679 grew into the Affaire des Poisons and ran for four years, taking in more than four hundred suspects and executing thirty-six people.
The figure this roster carries, one thousand to two thousand five hundred, is the most inflated number in this archive and needs saying plainly. It comes from testimony about infant remains buried or burned at her house, given by her daughter Marguerite and by associates before the Chambre Ardente, the special tribunal that heard the case. That tribunal used judicial torture, held its sessions in secret, and was gathering evidence about the king's mistress; every incentive in the room pointed towards larger numbers. What the evidence supports is a working abortion practice over many years, in a period when that work was capital, and the sale of poisons to people who then used them. What it does not support is a count.
She was convicted of witchcraft and poisoning and burned alive on the Place de Greve on February 22, 1680, having refused to make a public confession.
Louis XIV closed the tribunal in 1682, had the records concerning Montespan sealed and, by most accounts, personally burned some of them. That is why the case cannot be reconstructed: the archive that would settle what she did was destroyed by the person with most to lose from it.
Her entry belongs beside the Marquise de Brinvilliers in this archive. Brinvilliers was executed four years earlier, and it was her case that convinced the Paris authorities a poisoning network existed and set the investigation in motion that ended at La Voisin's door.