Saint-Lambert was born at Nancy and raised on his parents' estate at Affracourt, a village in Lorraine near Haroué, a seat of the Beauvau family, with whom he had close ties.

[citation needed] In 1739, Saint-Lambert joined the Heudicourt regiment in the Lorraine Guards, in which his boyhood friend, Charles-Just, prince de Beauvau-Craon, was already a colonel, despite being only 19 years old.

[citation needed] Saint-Lambert spent the winter quarter in Lunéville in 1745–46, and according to François-Antoine Devaux, he became at that time the lover of the Marquise de Boufflers.

[7] It was at this time that he gave himself the title Marquis de Saint-Lambert, to which he had no right; it was once claimed that he was not even of noble birth, but the evidence refuting that charge was published long ago.

This relationship became noteworthy because in 1757, while Saint-Lambert was away on military duty in the Seven Years' War, Jean-Jacques Rousseau suddenly conceived a mad passion for Sophie, which he wrote about in his Confessions.

"[13] Chateaubriand used the couple as symbols of a discredited era, when he wrote that they "both represented the opinions and the freedoms of a by-gone age, carefully stuffed and preserved: it was the eighteenth century expired and married in its manner.