From 1813 he was charged with the reorganization of the Italian troops into five divisions. In autumn 1813 he was given command of the 15th Division composed of four regiments and a divisional battery (brigades of Sant'Andrea and Moroni) of Italians in IV Corps under Henri Gatien Bertrand, replacing Luigi Gaspare Peyri, and saw action at the battles of Grossbeeren on 23 August, Dennewitz 6 September, Wartenburg on 3 October and Leipzig on 16–19 October. After Leipzig, Fontanelli's division successfully held Lindenau, allowing the remains of Napoleon's La Grande Armée to reach France.