Chapelain was a very successful fortune teller who had secured a fortune at her work. She had acquired her spouse, a former usher with a position as bureaucrat, and she also owned several buildings. In connection to the Poison Affair, she was implicated because she had formerly employed Françoise Filastre as a maid: when Filastre was arrested upon her return from a certain trip to the Auvergne (province) in December 1679, it was a trip which had been financed by Chapelain. Chapelain was also connected to Louis de Vanens, to whom she had rented a house. Adam Lesage claimed that Chapelain made her fortune by manufacturing poisons and performing black magic in collaboration with a man by the name of Boucher, and Filastre claimed that she had on occasion supplied Chapelain with poison for the use of selling, that Chapelain had commissioned her to help her form a pact with Satan, and that Chapelain also performed curses and other magical services for her clients. Filastre claimed, that it was Chapelain who had been commissioned by Madame de Montespan to place an assassin (Filastre), in the household of Angelique de Fontanges.
Like many other involved with the Poison Affair, Magdelaine Chapelain was never trialed, but imprisoned in perpetuity by a Lettre de cachet at Belle-Île-en-Mer. The date of death is not known for all of them, but of the prisoners whose date of death is known, Magdelaine Chapelain's death in June 1724 was the last recorded.