At first destined for a career as an architect (for which he showed a marked disposition), he fought in his first battles in 1788 as a volunteer in the Canaris (after its uniform's colour) cavalry regiment during the Brabant Revolution.

[2] After the revolution was stopped in 1790, he fled and offered the First French Republic his services, commanding a battalion of the Belgian Legion, fighting at Jemappes and rising to général de brigade in 1793 after his defence of the approaches to Lille against the young comte de Bouillé.

In 1796 he commanding the troops protecting the provinces of Groningen, Friesland and Drenthe, before being made military governor of the Hague.

During the Anglo-Russian invasion of Holland he brought up two-thirds of his 2nd Batavian division in forced marches from Friesland and he arrived on 8 September to take on a position in the center of the Franco-Batavian front, around Alkmaar, in time for the Battle of Krabbendam.

Dumonceau was then captured himself at Dresden on 11 November 1813 with maréchal Gouvion-Saint-Cyr, remaining a prisoner until Napoleon's abdication in April 1814.