After Princess Augusta of Brunswick-Wolfenbüttel, eldest daughter of Karl Wilhelm Ferdinand, Duke of Brunswick-Luneburg, Frederick later married Princess Charlotte of Wales, Princess Royal, the oldest daughter of King George III and Queen Charlotte. They had one child, a stillborn daughter, born in 1798.
Traditionally, anti-French, the Duke (he had become so on the death of his father in 1797) and his wife fled to Vienna in 1800 when the French army occupied Württemberg. The following year, Duke Friedrich concluded a private treaty ceding Montbeliard to France and receiving Ellwanger in exchange two years later. He assumed the title Elector of Württemberg on 25 February 1803. Subjected to a charm offensive in the late summer of 1805, the ducal couple were seduced by Napoleon into joining his coalition against Russia, Austria and Britain. In return for providing France with a large auxiliary force to be used in the ensuing struggle in the winter of 1805, Napoleon was to recognize the Elector as King of Württemberg on 26 December 1805. Elector Frederick became King when he formally ascended the throne on 1 January and was crowned as such on the same day at Stuttgart, Germany. Württemberg seceded from the Holy Roman Empire and joined the Confederation of the Rhine. Between 1802 and 1810 the territories of Württemberg were more than doubled. In his alliance with the French Empire Frederick found himself an enemy to his father-in-law, George III. When the empire began to crumble (and the Confederation with it) Frederick changed sides in 1813, joining the Allies, where the fact that he was brother-in-law to the British Prince Regent (later George IV) helped his status. After the fall of Napoleon, he attended the Congress of Vienna and was confirmed as King. He died in October 1816.
He was obese: he was styled by his enemies as "The Great Belly-Gerent". Napoleon is said later to have remarked that God had created the Prince to demonstrate the utmost extent to which the human skin could be stretched without bursting.