The young nobleman was, from the first, a prime favourite at the French court, owing partly to the recollection of his father's devotion to France, but principally because of his own amiable and brilliant qualities. Queen Marie Antoinette, who had first met Fersen when they both were age 18 (January 1774), was especially attracted by the grace and wit of "le beau" Fersen, who had inherited his full share of the striking handsomeness which was hereditary in the family. It is possible that Fersen would have remained at Versailles, following the American war, but he was commanded by his own sovereign, then at Pisa, that he desired him to join his suite. Fersen accompanied Gustav III of Sweden in his tour of Italy and France, and returned home with him in 1784.
In 1785 Marie Antoinette would give birth to Louis-Charles, the first titular Duke of Normandy in centuries. Afterwards Louis XVI wrote in his journal that it had happened just as when "his own son" had been born. Some have claimed that Louis-Charles, later Dauphin of France, was the biological child of Marie Antoinette and Fersen. However, this is unlikely. Some have claimed that Louis XVI actually meant when "his first son" was born. Secondly, little Louis XVII was noted to resemble two members of the Bourbon family: his paternal uncle Charles X (Louis XVI's youngest brother) and his late grandmother, Princess Maria-Josefa (Louis XVI's mother). The claim that Fersen was the biological father of Louis XVII has been discounted by the child's recent biographer, Deborah Cadbury, and by Marie-Antoinette's biographer Antonia Fraser.
It is simply impossible to state how far the love affair went. It was difficult for the Queen of France to remain alone for long; she was always accompanied by others. Unless some original documents are uncovered between Marie Antoinette and Fersen, it is an open question whether their relationship was platonic or not. The color of Fersen's eyes and hair is reason for controversy, as Marie Antoinette's son was fair-colored (as was Marie Antoinette herself, who was blonde), while Fersen's eyes were supposedly hazel, because they are said to be green, blue and even dark brown in different descriptions. The color of Fersen's hair is disputed. It is believed it was naturally light brown or a kind of dark blond; however, it is said he used to dye it black.
When Gustav III's war with Russia broke out, in 1788, Fersen accompanied his monarch as an adjutant to Finland, but in the autumn of the same year was sent to France, where the political horizon was already darkening. It was necessary for Gustav III to have an agent thoroughly in the confidence of the French royal family, and, at the same time, sufficiently able and audacious to help them in their desperate straits, especially as he had lost all confidence in his accredited minister, the Baron Erik Magnus Staël von Holstein. With his usual acumen, he fixed upon Fersen, who was at his post early in 1790. Before the end of the year he was forced to admit that the cause of the French monarchy was hopeless so long as the king and queen of France were nothing but captives in their own capital, at the mercy of an irresponsible mob.
Count von Fersen was instrumental in the writing of the Brunswick Manifesto.
He had the leading role in the royal family's flight to Varennes. He found most of the requisite funds at the last moment. He ordered the construction of the famous carriage for six, in the name of the Baroness von Korff, and kept it at his own home on the Rue Matignon, so that all Paris might get accustomed to the sight of it. He was the coachman of the fiacre which drove the royal family from the Place du Carrousel to the Porte Saint-Martin, where the carriage awaited them. He accompanied them as far as Bondy, the first stage of their journey.