She was the daughter of the General and riksråd count Magnus Julius De la Gardie and the political salonist Hedvig Catharina Lilje, and sister of scientist Eva Ekeblad.

Through her marriage, she became the matriarch of one of the most powerful noble families in Sweden and a riksrådinna (spouse of a riksråd), securing her a position in the very elite of the Swedish aristocracy, and socialized in the circles of the royal court: Queen Louisa Ulrika once referred to her and Eva Ekeblad as her only true friends.

She was described as a close friend of Gustav III during his first years as a monarch: she belonged to the intimate circle of debate-partners with whom he stayed up late at night discussing,[3][page needed] and he found her intelligent and cultivated.

Because of a confusion of the time, Sophie von Fersen would have been forced to wait dressed in her evening gown in the royal antechamber in the company of professional ballet dancers, which her mother considered to be shameful of her noble status.

[1] The relationship of the von Fersen family and the royal house was to become strained with time, especially during the crisis of 1789, when her spouse became arrested as one of the leaders of the nobility in opposition to the king's Union and Security Act.