Saint-Just was born in Decize in 1767. Following the outbreak of the revolution in 1789, Saint-Just soon sided with it, and was elected to the Legislative Assembly in 1791. He also addressed the National Convention at the trial of Louis XVI, stating "A king must reign or die".
By taking the position that Louis should be sentenced to death, Saint-Just was the first to voice a popular opinion that few dared to admit to. Saint-Just was strongly applauded by the Convention and soon became a noted revolutionary figure among the Jacobins.
In September 1793, the Reign of Terror began after the Committee of Public Safety became increasingly powerful. Saint-Just led the Committee along with Maximilien de Robespierre, Georges Couthon and Jean-Marie Collot d'Herbois and became a critic of Georges Danton, believing him to be too moderate.
Around this time, Saint-Just attempted to have the Marquis de Sade imprisoned, believing him to be an immoral influence on the citizens of the French Republic. To accomplish this, he intended to frame de Sade for murder. Saint-Just sent an agent to plan the murder of one of de Sade's prostitutes, Suzanna.
De Sade's valet Olivier had come to hate his employer, and the agent took advantage of this. The agent paid Olivier to murder Suzanna, as Saint-Just did not want the Committee to suspect him. The Assassin Arno Dorian investigated the murder, discovered the plot to frame de Sade and had Olivier imprisoned.
Possibly due to his extreme political positions, a legend grew alluding that Saint-Just had clothing made from human skin in the tanneries of Paris. These rumors caught the attention of Arno, who was charged by Saint-Just's valet to speak to a tailor. The valet had ordered a coat for Saint-Just from the tailor, but the tailor refused to deliver it.
The tailor revealed to Arno that the coat was made of a mysterious material, and that he had been turned away by thugs when he attempted to inquire the tanner about it. Infiltrating the tannery, Arno learned from the distraught tanner that he had been forced to produce leather for the coat from human skin. Arno vowed to bring Saint-Just to justice and told the valet about the leather.
The valet revealed that he was well aware of the matter, and when threatened by Arno, called his guards on him. Arno killed the guards and the valet, and returned to the tailor, informing him of the leather. Shocked, the tailor promised to use the leather as evidence in order to bring down Saint-Just, to which Arno wished him good luck.
With the eventual fall of the Jacobins, Saint-Just and his friend Robespierre were guillotined on 28 July 1794 as part of the Thermidorian Reaction.