In parallel to her career as a Ballerina, she was also a reputed courtesan. Just as her colleague Sophie Hagman, she became the lover of one of the princes: duke Charles, the later king Charles XIII of Sweden, and she is mainly remembered for this. She could not be officially seen at court as Hagman as she was a reputed courtesan, and neither was she the only lover of Prince Charles (who was talked about as having a harem), but she was known as his "Favourite Sultaness" and does seem to have had a great influence over him. She gave him a great interest in culture, made him write plays, and cheered him up when he was depressed. She also gave him an interest in champagne, which she liked herself. She is reputed to have been politically active: in 1790, she was rumoured to receive a pension from Charle's brother king Gustav III of Sweden in exhange for influencing his brother During the pilloring of Magdalena Rudenschöld (1794), who was judged guilty of treason, it was reported that people in the crowd was heard saying that Charlotte Slottsberg should have been standing on the platform instead of Rudenschöld, and her carriage was also attacked. She was allowed to use a carriage of a design normally reserved for women of the royal court, with guards in the duke's uniform, until 1795, when Charles saw it necessary to make their relationship less official.
She was Charles' most long-term mistress; their relationship, though not exclusive on either part, lasted for twenty years; from 1777 until 1797.