Charles was born on 6 March 1823 in Stuttgart as the son of King William I and his third wife Pauline Therese (1800–1873). As the king's eldest son he became Crown Prince of Württemberg.

He studied in Berlin and Tübingen.

On 13 July 1846 he married Grand Duchess Olga Nikolaievna of Russia, daughter of Tsar Nicholas I and Charlotte of Prussia. Charlotte was a daughter of Frederick William III of Prussia and Louise of Mecklenburg-Strelitz. She took the name Alexandra upon her marriage. Karl acceded to the throne upon his father's death in 1864.

The couple had no children, perhaps because of Karl's homosexuality. Karl became the object of scandal several times for his closeness with various men. The most notorious of these was the American Charles Woodcock, a former chamberlain whom Karl elevated to Baron Savage in 1888. Karl and Charles became inseparable, going so far as to appear together in public dressed identically. The resulting outcry forced Karl to renounce his favorite. Woodcock returned to America, and Karl found private consolation some years later with the technical director of the royal theater, Wilhelm George.

In 1870, Olga and Karl adopted Olga's niece Vera Konstantinovna, the daughter of her brother Grand Duke Konstantin.

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