He took an apprenticeship under Jacques-Louis David before working with Jean-Baptiste Isabey, seven years Guérin's junior, after meeting him in David's studio. David commissioned a portrait of one of his daughters from Guérin just before her marriage - Guérin accepted but only on the condition that David would pose his daughter for him. Marie-Antoinette became his main patron and protector. By 1792 he was a member of the Filles Saint-Thomas section of the National Guard, with whom he defended the royal family against the sans-culottes during the Demonstration of 20 June that year.