With the opening of the Southern theater in the American Revolutionary War, Guerard enlisted in the militia and participated in the siege of Charleston by the British in 1780. The colonists surrendered Charlestown on May 12, 1780 and Guerard became a prisoner, although he was paroled. A year later, Guerard was found in violation of his parole and jailed on the schooner "Pack Horse". He offered his estate to provide for the maintenance of the American prisoners, but the British refused this proposal because his land was in the territory controlled by the Americans. The British exiled him in 1781 to Philadelphia and Guerard subsequently made his way back to South Carolina.