On 16 December 1812, Mendizábal was named political chief of the Seigneury of Biscay. He called the acting Council of Biscay, who in the wake of the approval of a new Constitution in Cádiz sent a task force to the city with a negotiation mandate. In the final phases of the war, on 31 August 1813, he controlled one of the divisions in the bloody but definitive Battle of San Marcial. For his actions, he was decorated with the highest military honors of the Laureate Cross of Saint Ferdinand, and the Laureate of the Royal and Military Order of Saint Hermenegild.