He was the brother of another career soldier, General Abel Douay.
For bravery at the battles of Magenta and Solferino, he was elevated to brigadier general.
During the Franco-Prussian War he was Commander-in-Chief of the French Seventh Army Corps.
After the first defeats on the Alsace border, 7th Corps retreated and became part of Marshal Patrice de Mac-Mahon's frontline Army of Châlons.
After his return to France, Douay led his Fourth Army Corps against the Paris Commune.