Courtois and his brother Pierre learned the trade of making potassium nitrate for gunpowder for the French Revolution. Courtois, however, branched off from this venture to learn chemistry. Courtois lived at Saint-Medard Nitrary until he was about eighteen, when he left his family home to begin his trade apprenticeship in chemistry in Auxerre. Here for three years he was a student of M. Frémy, the future grandfather of Edmond Frémy. He then obtained a position with Antoine-François de Fourcroy at the École Polytechnique in Paris. In 1799 Courtois served as a pharmacist in military hospitals. In 1801 he returned to the "École Polytechnique" to work in the laboratory of Louis Jacques Thénard.