Decouz was born on 18 July 1775 in Annecy in the Kingdom of Sardinia but after the Duchy of Savoy was annexed to France, he joined the Republican French army as a volunteer. In March 1793 he enrolled in the 2nd Battalion of the Volunteers of Mont Blanc and became a sous-lieutenant two months later. He fought at the Siege of Toulon later in the year. After its conclusion, he was posted to the 19th Infantry Demi-brigade of the Army of Italy. In 1795 the 19th fought at the Battle of Loano. In 1796, the 19th became the 69th Line Infantry Demi-brigade and the unit fought at the Battle of Mondovì, Battle of Castiglione and the Siege of Mantua.