Born between 1590 and 1600 at Büttgen in the duchy of Jülich. His parents belonged to the numerous class of the lesser nobility, and at an early age he left home to follow the career of a soldier of fortune in the Walloon cavalry of the Spanish service. 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 terrible leader of cavalry forays. His services were even more conspicuous in the great pitched battle of Nordlingen (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 as well as the sword. In 1635 and 1636 his forays extended into Lorraine and Luxemburg, after which he projected an expedition into the heart of France. Starting in July 1636, from the country of the lower Meuse, he raided far and wide, and even urged the cardinal infante, who commanded in chief, to "plant the double eagle on the Louvre." Though this was not attempted. Worth's horsemen appeared at Saint-Denis before the uprising of the French national spirit in the shape of an army of fifty thousand men at Compiegne forced the invaders to retire whence they had come. The memory of this raid lasted long, and the name of "Jean de Wert" figures in folk-songs and serves as a bogey to quiet unruly children. In 1637 Werth was once more in the Rhine valley, destroying convoys, relieving besieged towns and surprising the enemy's camps. In February 1638 he defeated the Weimar troops in an engagement at Rheinfelden, but shortly afterwards was made prisoner by Bernhard of Saxe-Weimar. His hopes of being exchanged for the Swedish marshal Horn were disappointed for Bernhard had to deliver up his captive to the French. The terrible Jean de Wert was brought to Paris, amidst great rejoicings from the country people. He was lionized by the society of the capital, visited in prison by high ladies, who marvelled at his powers of drinking and his devotion to tobacco. So light was his captivity that he said that nothing bound him but his word of honour. However, he looked forward with anxiety for his release, which was delayed until March 1642 because the imperial government feared to see Horn at the head of the Swedish army and would not allow an exchange.
When at last he reappeared in the field it was as general of cavalry in the imperial and Bavarian and Cologne services. His first campaign against the French marshal Guebriant was uneventful, but his second (1643) in which Count Mercy was his commander-in-chief, ended with the victory of Tüttlingen, a surprise on a large scale, in which Werth naturally played the leading part. In 1644 he was in the lower Rhine country, but he returned to Mercy's headquarters in time to take a brilliant share in the battle of Freiburg. In the following year his resolution and bravery, and also his uncontrolled rashness, played the most conspicuous part in deciding the day at the second battle of Nordlingen. Mercy was killed in this action, and Werth succeeded to the command of the defeated army, but he was soon superseded by Field-marshal Geleen. Johann von Werth was disappointed, but remained thoroughly loyal to his soldierly code of honour, and found an outlet for his anger in renewed military activity. In 1647 differences arose between the elector and the emperor as to the allegiance due from the Bavarian troops, in which, after long hesitation, Werth, fearing that the cause of the Empire and of the Catholic religion would be ruined if the elector resumed control of the troops, attempted to take his men over the Austrian border. But they refused, to follow, and escaping with great difficulty from the elector's vengeance Werth found a refuge in Austria. The emperor was grateful for his conduct in this affair, ordered the elector to rescind his ban, and made Werth a count. The last campaign of the war (1648) was uneventful, and shortly after its close he retired to live on the estates which he had bought in the course of his career, and on one of these, Benatek near Königgratz, he died on January 16 1652.