Son of a sous-aide-major of the regiment de Conti-cavalerie. Saint-Hilaire was born in 1766 in Ribemont, in the Aisne department. From the age of eleven, he served as a cadet in his father's regiment, remaining there until 1783. The following year he was appointed sub-lieutenant in the Regiment d'Aquitaine and left with them for the Antilles. Becoming lieutenant in 1788 and captain in 1792, Saint-Hilaire advanced rapidly, despite being noble. He was appointed Chef de brigade (colonel) in 1794, Général de brigade in 1795, and then Général de division in 1799. He served at the camp in Saint-Omer in 1803 and during the campaigns of 1805, 1806 and 1807 he commanded the first division of the 4e corps (Soult). At the battle of Austerlitz, it was Saint-Hilaire who, with Vandamme's division, led the assault on the Pratzen Heights. He was wounded there in the head, but nevertheless won for himself the Grand-Aigle of the Légion d'honneur. He fought at Jena and at Eylau where his division performed remarkably well, including: the forward movement of his artillery (commanded by Séruzier, « le père aux boulets », who mentions this event in his memoirs) and the heroic resistance of the whole division, making it possible for Davout's 3e corps to arrive in time to ensure the victory.
He was made Comte d'Empire in 1808 and, in 1809, he commanded a division in Lannes' 2e corps. At Essling (22 May, 1809), his foot was blown off by a cannon ball, and he was taken to Vienna where he diedon 3 June. Speaking of Lannes and Saint-Hilaire, both of whom died during this latter campaign, Napoleon was to say on Saint Helena: "Those two would never have been unfaithful to the glory of the people of France."