Jacob was born in Anjou during the last years of the 16th century. He was enrolled as one of the pages to the duc de Guise, but he ran away to join some strolling players, assuming the stage name of Montfleury. About 1635, he was a valued member of the company at the Hôtel de Bourgogne, and he was in the original cast of the Cid (1636) and of Horace (1640).
He was the author of a tragedy, La Mort d'Asdrobal, performed in 1647. His son, Antoine Jacob, who was also known as Montfleury, was a rival of Molière and Cyrano de Bergerac. Both father and son feuded with Molière. Zacharie was mocked for his girth in a Molière play, to which Antoine responded with one mocking Molière. Shortly before his death, Zacharie denounced Molière to King Louis XIV as having committed incest, a rumor which has never gone away.
Montfleury died in Paris from the rupture of a blood-vessel, while playing the part of Orestes in Andromaque, in December 1667. It is believed that a metal belt he used to support his enormous belly was the cause.