In 1644, Mercy opposed the French armies, now under the Great Condé and the Vicomte de Turenne. He captured Freiburg and subsequently held it in an inconclusive but bloody battle in August. On 2 May 1645, he defeated Turenne at Mergentheim. Later in 1645, fighting once more against Condé and Turenne, Mercy was killed by a musket ball at the Battle of Nördlingen (or Allerheim) on 3 August while leading reinforcements to the focus of the action. On the spot where he fell, a memorial stone with the inscription Sta, viator, heroem calcas! () was set up, allegedly at Condé's instruction. His body was first brought to Donauwörth on an artillery wagon and then to Ingolstadt the next day, where he was buried in the Church of Saint Maurice. The French Marshal Gramont, who had been captured at the battle, reports of the exuberant reception that Mercy received from the people of the city who had hurried to the gates. He was very popular and highly regarded in Ingolstadt, where he had been the fortress commander and governor.
