Born as Jacques d'Adelswärd, he is related on his paternal side to
Axel von Fersen, a
Swedish Count who had a relationship with
Marie Antoinette. D'Adelswärd took on the name Fersen later in his life out of admiration for the distant relative.
D'Adelswärd-Fersen's grandfather had founded the steel industry in
Longwy-Briey, which was profitable enough that it made d'Adelswärd-Fersen exceedingly wealthy when he inherited at age 22. Consequently, he was much sought-after in the higher circles, as families hoped to marry him to one of their daughters.
Apart from joining the military, d'Adelswärd-Fersen already traveled extensively and settled down as a writer. He published several volumes of poems, for instance
Chansons Légères, and novels.
As a young man in the beginning of his twenties, his
homosexual leanings became apparent to him, which are also relatively clearly addressed in his poetry. Unfortunately, he was not sexually interested in adult men (which at the time in France would not have brought him into legal trouble) but in teenage boys between about 15 and 17 years old, i.e. he preferred
pederastic relationships. This inclination eventually caused his undoing in French society.