From a young age Koniecpolski chose to follow a military career. In 1610, together with his brother Przedbór, he took part in the Dymitriads against Muscovy, raising a group of 300 men to join the Army of the Polish–Lithuanian Commonwealth at Smolensk. On 4 July 1610 he participated in the Battle of Klushino before joining the Siege of Smolensk where Przedbór was crushed to death by the fortress' collapsing walls on 8 July 1611. After returning to his family mansion at Koniecpol with his brother's body, the following autumn Koniecpolski rejoined the army. Under the command of Grand Lithuanian Hetman Jan Karol Chodkiewicz, he took part in the effort to relieve and supply the besieged Polish forces in the Moscow Kremlin. During that time, he was entrusted by the Hetman with command of the right flank of the Polish forces.