In March 1680, Joly was arrested in connection to the Poison Affair, when she was pointed out by Etienne Guibourg.
Marguerite Joly became the star witness in the case against Francoise de Dreux.
Dreux had been acquitted in April 1680, but was arrested a second time because of the testimony of Joly.
During the torture, she confessed to several murders and pointed out Anne Meline as her accomplice.
She claimed that she had been present at the sacrifice of the nephew of La Poignard to Satan; at the sacrifice of an infant at a black mass for a Mademoiselle de Saint-Laurens to induce a desired marriage; that she had committed murder supplied with poisons by a female cosmetic in collaboration with Meline, who had also murdered Joly's former spouse.