Leclerc started his military career in 1791 during the French Revolution as one of the army volunteers of Seine-et-Oise and passed through the ranks of sous-lieutenant in the 12th Cavalry, then aide-de-camp to general Lapoype. He was made a captain and divisional chief of staff during the siege of Toulon, at which he first allied himself to Napoleon Bonaparte. Following the revolutionary success there, he campaigned along the Rhine. He began serving under Napoleon Bonaparte in the Alpine and Italian campaigns, fighting at Castiglione della Pescaia and Rivoli and rising to général de brigade in 1797. He was then charged with announcing to the French Directory the signature of the peace preliminaries at Leoben. Pauline Bonaparte was at this time receiving a large number of suitors, thus pressing her brother Napoleon Bonaparte to have her married off. On Leclerc's return, he accepted Bonaparte's offer of Pauline's hand in marriage and they married in 1797, having one child, Dermide, and occupying the Château de Montgobert.