[4] After the accession of Gustav III and the coup d'état which introduced absolute monarchy, his brother terminated these plans against their mother's will in October 1772, and began negotiations for a marriage between Charles and his cousin Hedwig Elizabeth Charlotte of Holstein-Gottorp.

He 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.

He was reportedly a client of the fortune teller Ulrica Arfvidsson, and he also favored the medium Henrik Gustaf Ulfvenklou.

[5] 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 Gustav III.

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 (7 June 1788) and Öland (26 July 1789).

[1] The autumn of 1789, Hedvig Elisabeth Charlotte wished to depose Gustav III and place her husband Duke Charles upon the throne.

[12] 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.

[12] Charles was in close connection to the opposition against Gustav III, and it is debated whether he knew of and supported the plans to assassinate the king.

When he was dying, he altered the will, and while still appointing Charles regent of his minor son, he was no longer to rule absolute, but restricted by a government consisted of the supporters of Gustav III.

[5] The Duke-regent was in practice not willing or capable to manage the state affairs, reportedly because of his lack of energy and staying power.

[1] Reuterholm ruled as the uncontested regent de facto the entire tenure of the regency, "only seldom disturbed by other influences or any personal will of charles".

The marriage negotiations of the young king disturbed the relationship to Russia, and the alliance with revolutionary France was greatly disliked by other powers.

The mystic Karl Adolf Boheman (1764–1831) had been introduced to the couple by Count Magnus Stenbock in 1793 and gained great influence by promising to reveal scientific secrets about the occult.

Boheman inducted them into a secret society Yellow Rose in 1801, where both sexes were accepted as members, and to which the Counts and Countesses Ruuth and Brahe as well as the mother of the queen were introduced.

[5] He kept passive during the Coup of 1809, and accepted the post of regent from the victorious party after having assured himself that the deposed monarch was not in mortal danger.

[5] On 13 March 1809, those who had dethroned Gustav IV Adolf appointed Charles regent, and he was finally elected king by the Riksdag of the Estates.

Charles's condition deteriorated every year, especially after 1812, and he eventually became but a mute witness during the government councils chaired by the crown prince, having lost his memory and no longer being able to communicate.