King Gustaf VI Adolf was the last who used the style "By the Grace of God King of the Swedes, the Goths/Geats and the Wends" (med Guds Nåde Sveriges, Götes och Wendes Konung; Latin: Dei Gratia Suecorum, Gothorum et Vandalorum Rex). This traditional title had been in use since the establishment of the hereditary monarchy in 1544. Carl XVI Gustaf instead chose the plain and simple title "King of Sweden" (Sveriges Konung), thereby ending a centuries-old tradition.

In the 16th century, Johannes Magnus construed a mythical line of Swedish kings, beginning with Magog, the son of Japheth, in an attempt to substantiate the antiquity of the Swedish throne. Based on that list, King Charles IX (reigned 1604 to 1611) adopted an ordinal unsupported by reliable historical sources. The only two previous monarchs named Charles (Karl in Swedish) have traditionally been numbered by counting backwards from Charles IX, and subsequent monarchs by counting forward from him. Adhering to that tradition, the current King of Sweden proclaimed himself Carl XVI Gustaf even though he is only the tenth Swedish monarch by the first name.

When Carl Gustaf ascended the throne, plans were already in place to replace the 1809 Instrument of Government, which made the King de jure chief executive. Though the King was a near-autocrat on paper, the Riksdag's authority grew steadily into the early 20th century. In 1914, Gustaf V made a speech which resulted in what is known as the Courtyard Crisis (Swedish: Borggårdskrisen) wherein he was accused of interfering with politics. With the principle of parliamentary democracy formally established since 1917, the King's actual direct involvement in policy-making lessened and the powers assigned to him were increasingly done by ministers in his name, notwithstanding the Instrument's stipulation that "the King alone shall govern the realm."

He was baptised in Charles XI's baptismal font, which stood on Gustav III's carpet and he lay in Charles XI's cradle with Oscar II's crown beside him. The same christening gown in white linen batiste which the Prince carried had been worn by his father in 1906 and would later be worn by his three children. His godparents were the Crown Prince and Crown Princess of Denmark (his paternal uncle and aunt), the Crown Prince of Norway, Princess Juliana of the Netherlands, the King of Sweden (his patrilineal great-grandfather), the Hereditary Prince of Saxe-Coburg and Gotha (his maternal uncle), the Crown Prince and Crown Princess of Sweden (his paternal grandfather and step-grandmother), and Count Folke and Countess Maria Bernadotte af Wisborg.

Carl Gustaf was born on 30 April 1946 at 10:20 in Haga Palace in Solna, Stockholm County. He was the youngest of five children and the only son of Sweden's Prince Gustaf Adolf and Princess Sibylla. He was christened at the Royal Chapel on 7 June 1946 by the Archbishop of Uppsala, Erling Eidem.

Prince Carl Gustaf was also given the title of the Duke of Jämtland. His father, Prince Gustaf Adolf, Duke of Västerbotten was killed in an airplane crash on 26 January 1947, at Copenhagen Airport. His father's death had left the nine-month-old Prince second in line for the throne, behind his grandfather, then Crown Prince Gustaf Adolf. When his great-grandfather Gustaf V died in 1950, the four-year-old Prince became the heir apparent of Sweden.

After graduating from high school, Carl Gustaf completed two and a half years of education in the Royal Swedish Army, the Royal Swedish Navy, and the Royal Swedish Air Force. He received his commission as an officer in all three services in 1968, and he eventually rose to the rank of captain (in the army and air force) and lieutenant (in the navy), before he ascended to the throne. He has also completed his academic studies in history, sociology, political science, tax law, and economics at Uppsala University and Stockholm University.

So empowered as head of his house, and with the support and authorization of the Swedish government, King Carl Gustaf since he was enthroned in 1973 has made a number of personal decisions regarding the titles and positions of relatives and family members, including the demotion of a sister, elevation of several commoners to nobility, rebuff of an elderly uncle’s wishes and the creation of new Swedish titles and duchies.

King Carl Gustaf holds the highest ranks in the three branches of the Swedish Armed Forces; this is due to the fact that he was, as stipulated by §14 of the 1809 Instrument of Government in effect at the time of his accession to the throne in 1973, the Commander in Chief (Swedish: Högste Befälhavare; not to be confused with the military professional holding the position of Supreme Commander) and therefore he was promoted ex officio from his earlier ranks of captain (Army & Air Force) and lieutenant (Navy), to general and admiral. Under the provisions of the Instrument of Government of 1974, which became effective on 1 January 1975, the King no longer holds this constitutionally-mandated position, but he kept his ranks à la suite since he no longer has any military command authority, except over His Majesty's Military Staff.

The King married Silvia Sommerlath, whose father was German and whose mother was Brazilian, and who had grown up in both countries. They met at the 1972 Summer Olympics in Munich, where she was an interpreter and host. The wedding was held on 19 June 1976 at Stockholm Cathedral and the ceremony was performed by the Archbishop of Uppsala, Olof Sundby. The wedding was preceded, the evening before, by a Royal Variety Performance, where the Swedish musical group ABBA performed "Dancing Queen" for the very first time, as a tribute to Sweden's Future queen. The King and his family moved to Drottningholm Palace west of Stockholm in 1980. He and the Queen have maintained their Business offices at the Royal Palace of Stockholm.

Prince Carl Philip was born the heir apparent. However, a constitutional reform, which was already under way at the time of his birth, made his elder sister, Victoria, the heir apparent and Crown Princess of Sweden on 1 January 1980, according to the principles of absolute primogeniture, which Sweden was the first recognised monarchy to adopt. King Carl Gustaf objected after the reform, not to the succession by females but to the fact that his son lost the position and title which he had had since birth.

Carl Gustaf was seven years old before he was told about his father's death, and he expressed his feelings about growing up without knowing his father in a speech in 2005.

The King is the honorary chairman of the World Scout Foundation, and often participates in Scout activities both in Sweden and abroad. He regularly visits World Scout Jamborees, for instance the 1979 Dalajamb World Jamboree International Encampment hosted by Sweden, the 2002 World Jamboree held in Sattahip, Thailand, and the 100th Anniversary of World Scouting 2007 World Jamboree held in Hylands Park, England. He also attended the 1981 National Scout Jamboree in Virginia, United States, and was awarded the Bronze Wolf, the only distinction of the World Organization of the Scout Movement, awarded by the World Scout Committee for exceptional services to world Scouting, in 1982. He also attended the 22nd World Scout Jamboree. He gave a speech on 6 August 2011 at the closing ceremony with more than 40,000 people watching. The band Europe also performed for him singing "The Final Countdown". King Carl Gustaf made an appearance at the 2013 Boy Scouts of America National Jamboree in West Virginia. Together with King Abdullah of Saudi Arabia, King Carl Gustaf has supported the Messengers of Peace programme.

The King is passionate about the environment, Technology, agriculture, trade, and industry. Like many members of the Swedish royal family, the King has a keen interest in automobiles. He owns several Porsche 911s—a car model which is said to be a particular favourite of the King—as well as a vintage Volvo PV444, a Ferrari 456M GT, an authentic AC Cobra and other cars. The first pictures taken of him and his Future wife were of them sitting in his Porsche 911. In the summer of 2005 he was involved in a traffic accident in Norrköping. The accident was described as a "fender bender", with no serious personal injuries claimed. Nevertheless, the incident caused national headlines. The King and Queen of Sweden frequently travel to the Summer and Winter Olympic Games, including in 2014, 2016 and 2018.

Carl Gustaf is the second-longest-reigning monarch in Swedish history after King Magnus IV, having overtaken his great-grandfather, Gustaf V, in 2016.