He is the youngest child and only son of Prince Gustaf Adolf, Duke of Västerbotten, and Princess Sibylla of Saxe-Coburg and Gotha. His father died on 26 January 1947 in an airplane crash in Denmark when Carl Gustaf was nine months old. Upon his father's death, he became second in line to the throne, after his grandfather, the then Crown Prince Gustaf Adolf. Following the death of King Gustaf V in 1950, Gustaf Adolf ascended the throne and thus Carl Gustaf became Sweden's new crown prince and heir apparent to the throne at the age of four.
A short while after he became King in 1973, the new 1974 Instrument of Government took effect, formally stripping him of any formal role in the legislative process, and several other duties normally accorded to a head of state, such as the formal appointment of the prime minister, signing off legislation, and being commander-in-chief of the nation's military. The new instrument explicitly limits the king to ceremonial functions and, among other things, to be regularly informed of affairs of state. As head of the House of Bernadotte Carl Gustaf has also been able to make a number of government-supported decisions about the titles and positions of its members.
The king's heir apparent, upon passage on 1 January 1980 of a new law establishing absolute primogeniture (the first such law passed in European history), is Crown Princess Victoria, the eldest child of the King and his wife, Queen Silvia. Before the passage of that law, Crown Princess Victoria's younger brother, Prince Carl Philip, was briefly the heir apparent, from his birth in May 1979.
Carl XVI Gustaf is the longest-reigning monarch in Swedish history, having surpassed King Magnus IV's reign of 44 years and 222 days on 26 April 2018.