He first fought on the side of the Swedish Empire under his second cousin, was captured, changed allegiance and raised a navy for General Albrecht von Wallenstein.
[1] In 1622, Ernst von Mansfeld's troops faced the army of Johann Tserclaes, Count of Tilly at the Battle of Mingolsheim.
Over the following year he "brought together, in fact, a not insignificant naval force"[1] consisting of various vessels either bought from local merchants and converted, or captured and reconfigured.
The Danes tried to halt Mansfeld's progress by attacking Wismar in April 1628 and again in June, each time blocking the harbour entrance with barges and bombarding the town with cannon fire from larger ships.
The Danes moved to nearby Poel where they captured several Wismarsche fishermen, took them to Denmark, and imprisoned them in Copenhagen until the town paid a ransom of 100 guilders per captive.
[4] After the Treaty of Lübeck and the withdrawal of the Danes, Mansfeld's navy continued to defend the Baltic Sea coastline of Germany against the forces of the Swedish Empire but fighting was limited to small skirmishes.
Mansfeld paid a portion of the claim and records to suggest he remained held in high regard for his skills as a commanding officer.
After the Battle of Nördlingen to the South, Mansfeld was given command of a smaller force[Note 4] which he took, in 1635, to Braunfels and then to Driedorf to counterattacks and looting by Protestant troops and mercenaries.