His two main philosophies were socialism and nationalism; by 1908 at the latest, after years of uneasy agnosticism, he had become a believing (but generally non-practicing) Roman Catholic.

His father Désiré Péguy was a cabinet maker, who died in 1874 as a result of combat wounds.

Péguy studied at the Lycée Lakanal in Sceaux, winning a scholarship at the École normale supérieure (Paris), where he attended notably the lectures of Henri Bergson and Romain Rolland, whom he befriended.

Around 1910 he fell deeply in love with Blanche Raphael, a young Jewish friend; however, he was faithful to his wife.

From 1900 until his death in 1914, he was the main contributor to and the editor of the literary magazine Les Cahiers de la Quinzaine, which at first supported the Socialist Party director Jean Jaurès.

However, Péguy ultimately ended this support after he began viewing Jaurès as a traitor to the nation and to socialism.

In the Cahiers, Péguy published not only his own essays and poetry, but also works by important contemporary authors such as Romain Rolland.

When the First World War broke out, Péguy became a lieutenant in the 19th company of the French 276th Infantry Regiment.