Leclerc started his military career as a volunteer in the French Revolution and within two years had risen to a post of divisional chief of staff at the siege of Toulon. Following the revolutionary success there, he campaigned along the Rhine. He began serving under Napoleon Bonaparte in the Italian campaign and fought at Castiglione della Pescaia and Rivoli.
In 1797, the newly promoted General de Brigade Leclerc married Napoleon's younger sister Pauline Bonaparte, with whom he had a child.
After tenures in the Army of Ireland and the Army of England, Leclerc gained promotion to general de division, which allowed him to aide Napoleon Bonaparte's bid for power. He participated in the coup d'etat of 18 Brumaire (in November 1799) making Napoleon the ruler and military dictator ("First Consul") of France. More military campaigns followed on the Rhine and in Portugal and then in 1802 his brother-in-law appointed him commander of the expedition to recover the former French colony of Saint-Domingue, now Haiti, where the Black general Toussaint L'Ouverture had mastered a virtually autonomous state.
With a large expedition that eventually included 40,000 European troups, the French won several victories after severe fighting. Toussaint agreed to retire. Acting on Napoleon's surreptitious instructions, Leclerc later seized L'Ouverture during a meeting and deported him to France where he died while imprisoned at Fort-de-Joux in the Jura mountains in 1803.
This treacherous act swung the tide inexorably against French hopes. Native insurgents began to fight the French, who were weakened by an epidemic of yellow fever. Leclerc died of the fever in November 1802. He was succeeded in command by General Donatien Rochambeau, whose brutal racial warfare only succeeded in drawing more people to the rebel armies, including Black and Mulatto army officers like Jean Jacques Dessalines, Alexandre Pétion and Henri Christophe. In November 1803 Rochambeau admitted defeat and Dessalines proclaimed the independence of Haiti on January 1, 1804.