Lucien Charles Xavier Saint was born on 26 April 1867 in Évreux, Eure, where his father was a doctor.

He obtained a law degree in Paris, and began his career as a lawyer before entering the prefectorial service in 1896 as a chief of staff.

[citation needed] In 1902 he married the daughter of Georges Trouillot, the Minister of Commerce, and became chief of staff to his father-in-law.

It was during his residency that one of the last bastions of Berber resistance in Morocco joined the makhzen, that of the Aït Atta refugees in Jbel Saghro led by Sheikh Assou Oubasslam.

He joined the Democratic Left group, and sat on committees on Foreign Affairs, Colonies, Algeria and Air.