Charles was King of Württemberg from 25 June 1864 until his death in 1891. Charles I married Grand Duchess Olga Nikolaevna of Russia in 1846 and ascended to the throne in 1864. Despite their marriage, the couple had no children, likely due to Charles' homosexuality. Charles was involved in several scandals, including a close relationship with American Charles Woodcock. In 1870, the couple adopted Olga's niece, Vera Konstantinovna. Charles I aligned with Austria during the Austro-Prussian War but later sided with Prussia in the Franco-Prussian War, joining the new German Empire in 1870. He died childless and was succeeded by his nephew, William II.

The Kingdom of Württemberg was a German state that existed from 1805 to 1918, located within the area that is now Baden-Württemberg. The kingdom was a continuation of the Duchy of Württemberg, which existed from 1495 to 1805. Prior to 1495, Württemberg was a county in the former Duchy of Swabia, which had dissolved after the death of Duke Conradin in 1268.