Pierre was brought up by his uncle, Antoine Séguier, president and mortier in the parlement, and became master of requests in 1620.

In this capacity he showed great independence with regard to the royal authority; but when in 1633 he became keeper of the seals under Richelieu, he proceeded to bully and humiliate the parlement in his turn.

He became allied with the cardinal's family by the marriage of his daughter Marie with Richelieu's nephew, Pierre César du Cambout, marquis de Coislin, and in December 1635 he became chancellor of France.

He put down pillage with a strong hand, and was sufficiently disinterested to refuse a gift of confiscated Norman lands.

His authority survived the changes following on the successive deaths of Richelieu and Louis XIII, and he was the faithful servant of Anne of Austria and of Mazarin.

His resolute attitude towards the parlement of Paris made the chancellor one of the chief objects of the hatred of the Frondeurs.

On the way he was assailed by rioters on the Pont-Neuf, and sought refuge in the house of Louis Charles d'Albert, duc de Luynes.

In 1666 Séguier was placed at the head of a commission called to simplify the police organization, especially that of Paris; and the consequent ordinances of 1667 and 1670 for the better administration of justice were drawn up by him.

In December 1642 he succeeded Richelieu as official "protector" of the Académie française, which from that time until his death held its sessions in his house.