At the age of sixteen Montecuccoli began as a private soldier under his uncle, Count Ernest Montecuccoli (died 1633), a distinguished Austrian general.
He fought in Pomerania, Bohemia and Saxony (surprise of Wolmirstadt, battles of Wittstock and Chemnitz), and in 1639 he was taken prisoner at Melnik and detained for two and a half years in Stettin and Weimar.