Because of the Holy Roman Empire's sanctions (Reichsexekution) against Gotha, his father lost his dominions and freedom on 15 April 1567.

The Principality consisted of southern and western areas of Thuringia, including the cities of Eisenach, Gotha and Hildburghausen.

The Veste Coburg was converted to a ducal fortress, and the arsenal as well as the government buildings (Stadthaus) were rebuilt at the Marktplatz (Market Square).

The choir of Morizkirche was gifted a huge Renaissance epitaph by John Casimir in 1598 in honor of his parents – a 12-meter-tall (39-feet-tall) alabaster monument with a richly sculptured series of pictures.

In addition, he founded the Gymnasium Casimirianum, extended the castle’s library to include his inheritance of book collections and in 1603 hired the composer Melchior Franck as the Hofkapellmeister (court master of music).

Politically, John Casimir succeeded in getting the Reichsritterschaft (Community of the Imperial Knights) to submit to his sovereignty by guaranteeing their jurisdiction over their country estates.

After he joined the Protestant side under Sweden, the Imperial and Bavarian troops under Wallenstein carried out the occupation of Coburg and the unsuccessful siege of the Veste in 1632.

In 1593 Duke John Casimir divorced his first wife for adultery and he then held her captive until her death at the Veste.