In his reign the Duchy lost its geopolitical position and became Sweden, Prussia and Russia's territorial subject of interests.

On 5 October 1675, in The Hague, Frederick Casimir married princess Sophie Amalie of Nassau-Siegen and returned home.

After his father died Frederick Casimir became the Duke of Courland and Semigallia and vigorously resorted to public affairs.

In the spring of 1697, Friedrich Casimir received a grand delegation from Russia, including the new Tsar Peter I, on whom the Jelgava Palace and its court left a great impression.

The duchy and Russia held diplomatic talks on mutual cooperation against the Swedes, but in the next winter of 1698, Frederick Casimir died at the age of 47.