At some point, Beylier was taken to Paris in order to be freed and joined the Assassins. After several years of service, he became a Master Assassin and took a seat on the Assassin Council of the French Brotherhood.
By 1776, Beylier was contacted by Eseosa, an Assassin of his native colony, who asked for support in the revolution for independence. As the French Assassins were preoccupied, Beylier was not able to provide reinforcements, but kept in contact with Eseosa nonetheless. Later, Beylier fought during the French Revolution, and eventually in Haiti, alongside Eseosa.
During his tenure as a Master Assassin, Beylier trained at least one apprentice, a man named Brasseur, and sent him to find the Roi des Thunes in the Cour des Miracles. Brasseur was unfortunately detected and killed, prompting the Council to send Arno Dorian in to finish the job. Arno later returned with news of his success, as well as a pistol taken from his target and which had belonged to Brasseur beforehand. Beylier paid his fallen apprentice his last rites before handing the pistol to Arno, telling him to use it with pride. Beylier was later incensed when Arno revealed that he had tracked and killed the Templar Chrétien Lafrenière without first informing or seeking permission from the Council, and voiced his opinion that Arno be removed from the investigation due to the risk of personal feelings clouding his judgment. Mentor Mirabeau disagreed, however, but Beylier was still doubtful. These doubts increased when Mirabeau was poisoned by fellow Master Assassin Pierre Bellec as a result of Arno proposing an alliance with exiled Templar Élise de la Serre, as well as when Arno slew Bellec in defense of Elise, leaving only Beylier and two others to lead the Council and the Brotherhood. When the Assassins discovered documents written by Mirabeau to the king, Arno was tasked with finding and destroying the royal copies of them before they put the Brotherhood at risk. Beylier explained that they were in the Tuileries Palace, which was besieged by revolutionaries after the Paris Commune declared open rebellion against the royals. Arno then left to do so.
After Arno had killed several Templars and attempted an attack against Grand Master François-Thomas Germain at the king's execution, he was brought before the remainder of the Council to answer for his actions. Supported by Masters Hervé Quemar and Sophie Trenet, Beylier lambasted Arno before Trenet expelled him in accordance with the Creed. Arno was later welcomed back, however, when it became clear that he had reformed and become wiser.