Rapp was born in the Koïfhus in Colmar, where his father worked as a janitor. He studied at the Protestant high school to become a pastor, but his hot temper led him to join the Cévennes Regiment in 1788. In 1793 he became a sergeant, in 1796 a captain and then an aide-de-camp to General Dessaix (1768-1800). He followed him to Italy and Egypt, where he became a colonel.
After the death of General Dessaix, whom he admired, Rapp became an aide-de-camp to General Bonaparte. Of great courage, as demonstrated by the many awards he received, he also made a faux pas: he once told the Emperor to beware of the Corsicans...
On the evening of the Battle of Austerlitz (1805), where he again made a name for himself, Rapp was promoted to major general. From 1807 to 1814 he was governor of Danzig and in 1809 was made count of the empire. He took part in the disastrous Russian campaign of 1812 and was injured while crossing the Beresina. Appointed first quartermaster general, Rapp was forced to capitulate in Danzig on November 29, 1813. Captured and taken to Kiev, he was only able to return to Paris after Napoleon's departure to the island of Elba. Louis XVIII made Rapp a peer of France and chamberlain of the king. However, in March 1815, Rapp met the emperor in the Tuileries, to whom he had always remained loyal. The emperor's death affected him so much that Louis XVIII had to console him himself: "Don't be shy, Rapp, I hope you will cry for me too."