He was a close friend of Maximilien Robespierre and served as his most trusted ally during the period of Jacobin rule (1793–94) in the French First Republic.
Saint-Just worked as a legislator and a military commissar, but he achieved a lasting reputation as the face of the Reign of Terror.
He publicly delivered the condemnatory reports that emanated from Robespierre and the Committee of Public Safety and defended the use of violence against opponents of the government.
He supervised the arrests of some of the most famous figures of the Revolution and saw many of them off to the guillotine.
For his unyielding severity, later writers dubbed him the "Angel of Death".
From its beginning in 1789, the Revolution enthralled the young Saint-Just, who strove to take a leading role.
He became a commander in his local National Guard unit.
Shortly after reaching the minimum legal age of 25 in August 1792, he won election as a deputy to the National Convention in Paris.
Despite his lack of record or influence, Saint-Just boldly denounced King Louis XVI from the speaker's rostrum and spearheaded a successful movement to have him executed.
His audacity brought him political recognition and the lasting favor of Robespierre.
Saint-Just joined him on the Committee of Public Safety and later served a term as President of the Convention.
Along the way he was a primary draftsman of radical Jacobin legislation such as the VentĂ´se Decrees and the Constitution of 1793.
Dispatched as an overseer to the army during its rocky start in the French Revolutionary Wars, Saint-Just imposed severe discipline.
At the same time, he ensured that the troops were protected by the new anti-aristocratic order promised by the Revolution.
He was credited by many for the army's revival at the front.
As the death toll mounted, opponents ultimately found their footing.
Saint-Just and Robespierre were arrested in the bloody coup of 9 Thermidor (27 July 1794) and executed the next day along with many of their allies.
In most histories of the Revolution, their deaths at the guillotine mark the end of the Reign of Terror and the beginning of a new phase, the Thermidorean Reaction.