Dumas served as a captain in the militia and helped defend the town of Quebec against the Americans in 1775; he remained active in the militia becoming lieutenant-colonel by 1802. In 1776, he married Marie-Françoise Fornel, the daughter of Louis Fornel and the widow of Antoine-Florent Meignot. In 1778, Dumas became sole lessee of the ironworks, holding it until 1783, when the lease was taken over by Conrad Gugy.
Dumas held a military position in the militia and assisted in defending Quebec against American forces in 1775; he continued to be involved in the militia, rising to lieutenant-colonel by 1802. He married Marie-Françoise Fornel in 1776, who was the daughter of Louis Fornel and the widow of Antoine-Florent Meignot. Dumas became the sole lessee of the ironworks in 1778, holding it until 1783, when Conrad Gugy took over the lease.