Carl was described as dependent and easily influenced. His numerous affairs gave him the reputation of being a libertine. He was reputed for his “harem” of lovers, of which the more well known were Augusta von Fersen, Charlotte Eckerman, Françoise-Éléonore Villain, Mariana Koskull and Charlotte Slottsberg, the last one reputed to have had political influence over him.
Carl unsuccessfully courted Magdalena Rudenschöld, and her refusal of his advances has been pointed out as the cause of the harsh treatment he exposed her to as regent during the Armfelt conspiracy. After the late 1790s, when his health deteriorated as a result of a series of rheumatic attacks, his relationship to his consort improved and she gained more influence over him.
The Duke was known for his interest in the supernatural and mysticism, and he was engaged in several secret societies. He was a member of the Freemasons. He was reportedly a client of the fortune teller Ulrica Arfvidsson, and he also favored the medium Henrik Gustaf Ulfvenklou.
Duke Carl was given several political tasks during his tenure as a duke. In 1777, he served as regent during Gustaf III’s stay in Russia. In 1780, he served as formal chief commander during the king’s stay in Spa. The same year, Gustaf III named him regent for his son should he succeed him while still a minor. However, he was not appointed regent during the journey of the king to Italy and France in 1783-84, and in the following years, he came under the influence of Gustaf Adolf Reuterholm, who was in opposition to the monarch, and came to be less trusted by Gustaf III.
In 1785, he was offered the Dukedom of Courland by the nobility of the Duchy and given the support of Gustaf III. This however never materialized.
On the outbreak of the Russo-Swedish War of 1788 he served with distinction as admiral of the fleet, especially at the battles of Hogland (June 7, 1788) and Öland (July 26, 1789). On the latter occasion he would have won a signal victory but for the remissness of his second-in-command, Admiral Liljehorn.
The autumn of 1789, Hedvig Elisabeth Charlotte wished to depose Gustaf III and place her husband Duke Carl upon the throne. Her ideal was the Swedish Constitution of 1772, which she saw as a good tool for an enlightened aristocracy, and the war and the Union and Security Act had made her a leading part of the opposition. She cooperated with Prince Fredrik Adolf of Sweden and Gustaf Adolf Reuterholm. The plan was to force Carl to act as a symbol of the opposition to the Union and Security Act when the time was right. When the time arrived to make Carl act, however, he refused, which effectively foiled the coup.
Carl was in close connection to the opposition against Gustaf III, and it is debated whether he knew of and supported the plans to assassinate the king.