After studying at the Irish College, Douai, he joined the National Guard of Dunkirk in 1789, at the start of the French Revolution, and was elected captain of a unit of volunteers in September 1791.

[2] Mortier was tasked by General Jacques Maurice Hatry to negotiate the surrender of the Fortress of Mainz,[3] which he completed successfully and then returned to Paris.

[2] His successful occupation of Hanover, bringing about the Convention of Artlenburg, led Napoleon to include Mortier in the first list of marshals created in 1804.

[7] When the War of the Fourth Coalition broke out in 1806, Napoleon ordered Mortier to assume command of the reformed VIII Corps on 1 October.

On 16 October, two days after his crushing victory over Prussia at Jena-Auerstedt, Napoleon ordered Mortier and Louis to conquer the Electorate of Hesse.

Mortier was to occupy Fulda and then the capital city of Kassel, rule as military governor, and imprison the Elector of Hesse, William I.

Every Hessian officer above the rank of lieutenant would be arrested and Napoleon stated his intention to "wipe the house of Hesse-Kassel from the map".

[citation needed] Mortier left a division to hold Hesse-Kassel while the rest of his corps was directed to mopping-up operations in Prussia.

[8] In 1808, Napoleon rewarded Mortier for his actions at Friedland with the title of "Duke of Treviso" (Duc de Trévise in French), a duché grand-fief (a rare, but nominal, hereditary honor, extinguished in 1946) in his own Kingdom of Italy.

[2] In October 1808, Mortier was sent to Spain in the campaign for the recapture of Madrid, at the head of the V Corps, which he led at the Battle of Somosierra and the Second Siege of Zaragoza.

[2] Mortier again commanded the Young Guard in several battles of the German campaign, including Lützen, Bautzen, Dresden and Leipzig.

[2] During the defense of France in 1814, he rendered brilliant services in command of rearguards and covering detachments,[5] and led the Old Guard at Montmirail, Craonne, Laon, and at the final Battle of Paris.