He served as Aide-de-camp to French Generals Louis Desaix and later Napoleon Bonaparte, whose life he saved on multiple occasions.
He began theological studies to become a clergyman, but with his build and heated character, he was better suited to the military, which he joined in March 1788.
He was made aide-de-camp of Louis Desaix, who named him captain and took him to Egypt, where Rapp distinguished himself at Sediman, capturing an enemy battery.
Rapp stayed in the line of fire: at Essling, he led the front of his fusiliers of the Garde impériale and carried the day; during the signing of the Treaty of Schönbrunn, he averted a planned attempt on Napoleon by the young Friedrich Staps.
He saved Napoleon's life a second time by repelling an attack of Don Cossacks near Maloyaroslavets and was again wounded at the passage of the Berezina, fighting alongside Ney in the rear guard.