Born in Morlaix, Brittany, in a family originating in the region of Quintin and having studied Breton in his youth, Fleuriot passed his university history agrégation in 1950.

He entered the Centre national de la recherche scientifique in 1958 and earned his doctorate at the Sorbonne University in 1964, defending a thesis called Le vieux-breton, éléments d'une grammaire (Old Breton, an Elementary Grammar), along with a complementary thesis, Dictionnaire des gloses en vieux-breton (Dictionary of Old Breton Glosses).

[citation needed] Fleuriot's book Les origines de la Bretagne defended a "two wave" model of British immigration into Brittany and argued that the legend of King Arthur arose from the life of Romano-British leader Ambrosius Aurelianus, who was known in Gaul as Riothamus.

Fleuriot came into conflict with François Falc'hun's claim that Breton was essentially native Gaulish, only influenced by the incoming British language.

[citation needed] A bibliography has been established by Gwennole ar Menn in Bretagne et pays celtiques, langues, histoire, civilisation (Skol, PUR, 1982), a collection of articles in honour of Léon Fleuriot.