Werth was born in 1591 at Büttgen in the Duchy of Jülich as the eldest son of lesser noble Johann von Wierdt († 1606) and Elisabeth Streithoven. He had eight brothers and sisters. At an early age he left home to become a soldier of fortune in the Walloon cavalry in the Spanish Netherlands. In 1622, at the taking of Jülich, he won promotion to the rank of lieutenant. He served as a colonel of cavalry in the Bavarian army in 1630. He obtained the command of a regiment, both titular and effective, in 1632, and in 1633 and 1634 laid the foundations of his reputation as a swift and fearsome leader of cavalry forays. His achievements were even more conspicuous in the great pitched Battle of Nördlingen (1634), after which the emperor made him a "Freiherr" of the Empire, and the elector of Bavaria gave him the rank of lieutenant field-marshal. About this time he armed his regiment with the musket in addition to the sword.