In 1791, he volunteered to join the French Royal Army, serving as a second lieutenant in the 12th Regiment of Chasseurs à Cheval before becoming an aide-de-camp to Jean François Cornu de La Poype.

[citation needed] Following the capture of Toulon, he served in the Army of the Rhine and fought in the Italian campaigns of the French Revolutionary Wars, participating in the battles of Castiglione and Rivoli and rising to the rank of brigade general by 1797.

[citation needed] Leclerc was later appointed as chief of staff under Louis-Alexandre Berthier and Guillaume Brune, participating in the second French expedition to Ireland led by Jean Joseph Amable Humbert in 1798.

After Napoleon returned from the French invasion of Egypt and Syria, he promoted Leclerc to the rank of divisional general and sent him back to the Army of the Rhine, then under Jean Victor Marie Moreau's command.

Leclerc was then appointed as commander-in-chief of a French army corps that Napoleon planned to send to Portugal to force the Portuguese government to renounce the Anglo-Portuguese Alliance, though that expedition never took place.

[citation needed] In 1791, enslaved Black people in the French colony of Saint-Domingue (modern-day Haiti) launched a slave rebellion which initiated the Haitian Revolution.

[2][3] In his initial instructions, Napoleon directed Leclerc to liquidate Louverture's government and deport his military officers to France, while publicly maintaining the abolition of slavery in Saint-Domingue.

The prospect of a similar restoration of slavery in Saint-Domingue swung the tide inexorably against French hopes of reimposing control, as Leclerc began summarily executing suspected conspirators en masse.

[citation needed] By October 1802, Leclerc wrote a letter to Napoleon advocating for a war of annihilation, declaring that "We must destroy all the blacks of the mountains – men and women – and spare only children under 12 years of age.

After Christophe massacred several hundred Polish soldiers at Port-de-Paix following his defection, Leclerc ordered the arrest of all remaining Black troops in French service in Le Cap, executing 1,000 of them by tying sacks of flour to their necks and pushing them off the side of ships.