He was the son of the statesman Axel von Fersen the Elder and the countess Hedvig Catharina De la Gardie (through her relatives to the Royal House of Vasa), nephew of Eva Ekeblad and grandson of General Hans Reinhold Fersen. He was carefully educated at home, at the Carolinum at Brunswick, in Turin, and in Strasbourg. In 1779 he entered the French military service with the Royal-Bavière regiment, accompanied General Rochambeau to America as his adjutant, served as interpreter between him and General Washington, distinguished himself during the war with the Kingdom of Great Britain, notably at the siege of Yorktown, 1781, and in 1785 was made proprietary colonel of the regiment Royal Suédois. He is famous as a lover and had affairs with various women, especially the Italian-born adventuress Eleanore Sullivan, and evidently the Royal Duchess Hedwig Elizabeth Charlotte of Holstein-Gottorp, married to the future King Charles XIII of Sweden; King Charles was in his turn the lover of the cousin of Axel, Augusta von Fersen.
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.
In the 1990s, Philippe Delorme, the contemporary authority on the subject, arranged for DNA testing of Louis XVII's heart. Ernst Brinkmann of Germany's Münster University and a Belgian genetics professor, Jean-Jacques Cassiman of the Katholieke Universiteit Leuven, conducted two independent tests. In 2000, comparison with DNA from the hair of Marie-Antoinette confirmed the heart was from a close relative of hers, and it was finally buried in the Basilica on June 8 2004.
It should be noted, however, that the DNA tested was mitochondrial DNA. This DNA is inherited only from the mother and allows tracing of a direct maternal genetic line. Assuming there was no tampering with the tests' samples, therefore, the comparison only proved that the two samples shared the same maternal ancestry. It does not prove that the heart belonged to a particular individual. Since there was a tradition of removing royal hearts after death, it is possible that the heart may have been that of another young royal, for instance that of Louis XVI's first son, Louis-Joseph-Xavier-François, who died in 1789. However, the heart of Louis-Joseph would have been removed and embalmed as was customary for all princes of France. The heart tested as part of the DNA experiment had not been embalmed, only preserved in alcohol. This is consistent with Pelletan's story of having left the heart in a jar of alcohol after removing it in 1795 from the body that was claimed to be that of Louis XVII.
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 Carrousel to the Porte Saint-Martin, where the carriage awaited them. He accompanied them as far as Bondy, the first stage of their journey.
During this congress it is told that Napoléon simply called him Monsieur, neglecting the fact that he was an appointed ambassador, a count, and one of the Lords of the Realm of Sweden (en av Rikets Herrar). Napoléon declared that he would never discuss anything with a person who had had an affair with the widow Capet (Marie Antoinette). Fersen thereupon served the following two years as Sweden's envoy to the court of Baden.
During the regency of the Duke Charles of Södermanland (1792-1796) Fersen, like other prominent Gustavians, or loyal supporters of the late Gustav III, was in disgrace. When Gustav IV Adolf attained his majority in 1796, Fersen was welcomed back to court. In 1799 he was made chancellor of Uppsala University to suppress radical student unrest and in 1801 was appointed Riksmarskalk, or Marshal of the Realm. On the outbreak of the war of the Third Coalition with Napoleon in 1805, Fersen accompanied Gustav IV Adolf to Germany as his political advisor. He prevented Gustav Adolf from attacking Prussia when it declined to join the war against France. During the rest of Gustav IV Adolf's reign Fersen was therefore in semi-disgrace, though he continued to serve in Sweden's interim governments when the king was abroad.
When on June 20, 1810, the prince's body was conveyed to Stockholm, Fersen, as Marshal of the Realm, received it at the barrier and led the funeral cortege into the city. His fine carriage and splendid dress seemed to spectators an open derision of the general grief. The crowd began to murmur and presently to throw stones and cry "murderer!" He was forced to seek refuge in a house near the Riddarhus Square, but the mob rushed after him, brutally maltreated him, tearing his clothes to pieces. At this time the Royal Life Guards standing in formation on the square attempted to protect Fersen, but the commanding officer gave the order "För fot gevär!" (English: Attention! Rifles by the foot).
To quiet the people and save the unhappy victim, two officers volunteered to conduct him to the court house and there place him in arrest. But he had no sooner been taken there than the crowd, which had followed him all the way beating him with sticks and umbrellas, broke in, dragged him out, and kicked and trampled him to death. The riot, which lasted more than an hour, happened, too, in the presence of numerous troops, who were forbidden to rescue the Marshal of the Realm from his tormentors. Later that evening the troops fired on the rioters, killing and wounding several of them.
In the circumstances, one must adopt the opinion of Fersen's contemporary, Baron Gustaf Armfelt, "One is almost tempted to say that the government wanted to give the people a victim to play with, just as when one throws something to an irritated wild beast to distract its attention. The more I consider it all, the more I am certain that the mob had the least to do with it. . . . But in God's name what were the troops about? How could such a thing happen in broad daylight during a procession, when troops and a military escort were actually present?"
The responsibility must ultimately rest with the government of Charles XIII of Sweden, which apparently intended to intimidate the Gustavians by a riot directed against one of their most conspicuous figures. The King is reported to have said the day before, "Det skadar inte om den förnäma herren får lite smuts i sin vagn." (English: "It won't hurt if that distinguished gentleman gets some dirt in his carriage"). Armfelt was absent at the time, so Fersen fell victim.
A few months after the murder Axel von Fersen and his family were cleared of any suspicion connected with the death of Carl August of Augustenburg, and he finally received a state burial with all pomp and ceremony. His sister Sofie Piper thereafter withdrew from Stockholm to her Löfstad manor, near Norrköping. Here she raised a memorial to her brother, with the inscription:
Åt en oförgätlig broder, mannamodet uti hans sista stunder den 20 juni 1810 vittna om hans dygder och sinnes lugn (To an unforgettable brother, the courage in his last moments on the 20th of June 1810, bears testimony to his virtues and clean conscience)