He participated in the 1805 Ulm Campaign, and was wounded in the battle of Dürenstein. In 1807, he was part of the French military mission to the Ottoman Sultan Selim III, tasked with shoring up the defences of Constantinople. Fabvier then managed to join the diplomatic mission of General Charles Mathieu Gardanne, Napoleon"s envoy to Persia, who tried to combat British and Russian influence in the region.
In 1809, he returned to Europe via Russia, and served for a while as a volunteer in the Polish army of the Duchy of Warsaw.
Arriving at Vienna, he was named captain in the French Imperial Guard. He served as aide-de-camp of Marshal Auguste de Marmont in Spain, and was sent by him to Russia to inform Napoleon on the Battle of Salamanca.
He arrived at Napoleon"s headquarters on 6 September 1812, the eve of the Battle of Borodino. Fabvier was gravely wounded in this battle, leading the charge during the final assault on the Russian fortifications.
Napoleon rewarded him by naming him artillery major in the VI Corps under Marshal Ney.
He distinguished himself further during the operations of the War of the Sixth Coalition in Germany, and was raised to colonel of the General Staff and made baron de l"Empire. He participated in the retreat into France, and on 31 March 1814, on behalf of Marshals Marmont and Mortier, he signed the surrender of Paris to the Coalition armies. During the Hundred Days, he joined the frontier defence forces as a volunteer.
After Napoleon"s downfall in 1815, he continued to serve in the royal French army.
In 1817, he accompanied Marshal Marmont as chief of staff in quelling the riots at Lyon, provoked by the harsh conduct of the local military governor, General Simon Canuel. Soon after, he was suspended from his military duties for his liberal beliefs, and was arrested in August 1820 and charged with participation in a military conspiracy.
Although he was released for lack of evidence, he was later called as a witness, but refused to disclose a name demanded by the public prosecutor, for which he was fined 500 francs. In 1822, he was charged with aiding the flight of four sergeants at Louisiana Rochelle, but was acquitted.
In 1823 he decided to leave France and went to Greece, to help the Greeks during their ongoing War of Independence.
His first task was the supervision of the fortifications of Navarino. Then he travelled to Britain to drum up support among the Philhellenes. Returning again to Greece, he was appointed head of the small Greek regular army, with which he participated in several battles, most notably the Siege of the Acropolis of Athens in 1826.
In 1828, he returned to France, only to return to Greece alongside the French Morea expedition.
In 1830, he returned to France and took part in the July Revolution. Initially chief of staff to General Étienne Maurice Gérard, on 4 August Fabvier was named military commander of Paris.
In 1831, he resigned his commission and retired with the rank Lieutenant General. Fabvier was made a peer of France in 1845, and in 1848, he was sent as the French ambassador to Constantinople, and thereafter to Denmark.
Back in France he was elected to the National Assembly of France as a representative of Meurthe.
There he sided with the conservative group of the assembly. He retired from public life on 2 December 1851, and died in Paris four years later.