Sobieski was a member of the Polish defense crew besieged by Cossacks at Zbaraż from 10 July to 22 August 1649. In 1650 he fought against the Cossacks at Kamieniec Podolski. Marek Sobieski also took part in the Battle of Beresteczko on 28–30 June 1651. After the Polish victory, due his bravery, he received a saber of Tugay Bey, who had been killed during the battle. In September 1651 he fought at the Battle of Biała Cerkiew. When the Poles, Cossacks and Tatars began negotiations, Marek Sobieski was sent to the Cossack camp as a guarantor of safety for the Cossack leader, Bohdan Khmelnytsky, who was in the Polish camp.