Fabian von Fersen served in the Royal Life Guards from 1771 and 1796 and climbed through the ranks from an ensign to a colonel lieutenant.
Before the deposition of Gustav IV Adolf during the Coup of 1809, he prevented the monarch from the withdrawal of funds from the national bank.
At that occasion, his brother Count Axel von Fersen the Younger was murdered in a riot suspected of having poisoned Carl August, and Fabian and his sister Sophie were also targeted by the mob as known followers of the Gustavian Party.
Fabian von Fersen demanded that the paper of his murdered brother be sealed, and him and his wife (at that point mistress of the Robes of the queen), both left court.
Hedvig Ulrika De la Gardie, whose interests in Sweden he protected when her spouse was exiled for treason, describes him as noble et généreux, and Anders Fredrik Skjöldebrand said of him that he had: "many good qualities, such as a pure and open personality, honesty and reliability, but also the haughty bearing of his family in his way of interaction".