Having become duke at the age of eighteen, Friedrich ruled for fifty-five years – longer than any other duke of Courland. He was a provident and focused politician, who managed to maintain the independence of Duchy of Courland in difficult circumstances. In 1594, according to his father’s will he assigned the Courland part of the Duchy to his brother Wilhelm. Since then the Duchy had two courts (Jelgava and Kuldīga) with split financial administration but common general administration and the court of law. When Wilhelm lost the throne, Friedrich achieved his brother’s acquittal as well as reparation of his nephew’s rights.
Friedrich died in Jelgava and was buried in the vault in February 1643. His pewter sarcophagus with the coats of arms of Courland and Pomerania-Wolgast was made by pewterer from Jelgava Franz Warnradt. The sarcophagus is partly restored.