As a younger son with little prospects he joined the Hannoverian army, becoming Field-Marshal as well as Governor of Hannover. He took part in the seven years war with credit. His elder brother, Adolf Friedrich IV, who at fourteen had become the ruling Duke of 
Mecklenburg-Strelitz, remained unmarried. However, in 1768, Karl married Friederike von Hessen-Darmstadt by whom he became the father of ten children. When she died in 1782, he married her younger sister, Charlotte, and fathered another son. 

In 1794 his brother died and he became the ruling Duke. For several years there was peace in Mecklenburg-Strelitz until 1806 when the French, in pursuit of the Prussians, committed attrocities on the population of his Duchy. It was only because of the intervention of the Bavarian king that Karl was allowed to remain as Duke, though Mecklenburg-Strelitz has to supply the French occupation forces. Karl was forced to join the Rhine confederation and, in 1812, supply soldiers for Napoleon's Russian campaign. In the spring of 1813 only a 
quarter returned. On 30 March 1813, Karl declared himself independant from Napoleon and called his people to arms. 

Under his youngest son, also named Karl, the Strelitz soldiers took part in the battles of Goldberg, Wartenburg and Mockern and proceeded as far as Paris. 
In 1815 the Congres of Vienna elevated Mecklenburg-Strelitz to a Grand Duchy with part of the Saar-region added to his territory. However, this area was sold in 1819 to Prussia for a million Thaler. Grand Duke Karl died in Neustrelitz on 6 November 1816.

Source: Leo van de Pas