Rapp was born in the Koïfhus in Colmar, where his father worked as a caretaker. He studied at the Protestant Gymnasium (high school) with the intention of becoming a pastor, but his fiery temper led him to join the Cévennes Regiment in 1788. In 1793, he became a sergeant, in 1796 a captain, and then adjutant to General Dessaix (1768-1800). He followed him to Italy and Egypt, where he rose to the rank of colonel.
After the death of General Dessaix, whom he admired, Rapp became adjutant to General Bonaparte. A man of great courage, as evidenced by the many accolades he received, he too occasionally committed a faux pas: once, he told the Emperor to beware of the Corsicans...
On the evening of the Battle of Austerlitz (1805), where he once again distinguished himself, Rapp was promoted to major general. From 1807 to 1814, he was Governor of Danzig and in 1809 was appointed Count of the Holy Roman Empire. He participated in the disastrous Russian campaign of 1812 and was wounded during the crossing of the Berezina River. 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 for the island of Elba. Louis XVIII made Rapp a Peer of France and Chamberlain to the King. However, in March 1815, Rapp met the Emperor, to whom he had always remained loyal, at the Tuileries Palace. The Emperor's death affected him so deeply that Louis XVIII himself had to console him: "Don't be shy, Rapp, I hope you will weep for me too."