The later “du Plessis” family may have lived in Pontus under King Mithdrates and after he was defeated by the Roman general in 66 BC, they Pompey accompanied Prince Sigge from Asia Minor via the present Russia to Norway and later went to France with the Viking leader Rollo and served him as one of his subordinate nobles. The first recorded member of the family is Osulf son of Frame, born about 961 Died about 1030. His son, Grimoult du Plessis, born about 1000 and died about 1047 who took part on a revolt against William I, Duke of Normandy (Later known as William the Conqueror) with some of William’s other noble vassals. The rebels lost a battle against William I, Duke of Normandy aided by Henry I, King of France, after which Grimoult was imprisoned in a dungeon at Rouen, where he soon died and was buried in his fetters. This was why he was considered a traitor of Valognes et de Val-és-Dunes.
The surname "du Plessis" comes from the fortified site of Plessis-Grimoult, (The bailey castle of Plessis-Grimoult, called Calvados is presented in the form of an oval curtain wall 50 to 60 meters in diameter, made of earth ramparts at the top of which stand the remains of a fortified gate and a small masonry tower.) it was built by Grimoult du Plessis.
Geoffroy du Plessis added "de Richelieu" to the surname after 1488, when his wife's brother Louis de Clérembault, Lord of Richelieu, knowing that his death was imminent and having no heir, made his nephew Geoffroy du Plessis, “his main and executory heir, only and for all”. The du Plessis family became the Lords of the Manor of Richelieu, and took the name of du Plessis de Richelieu.