L'Estourbeillon was associated with anti-Dreyfusard views, and attended a "grand reunion antisemite" in Nantes with Jules Guérin in 1898.
He is generally known as Régis de l'Estourbeillon, but held the title Marquis de l'Estourbeillon. He became interested in preserving Breton regional identity as a young man, founding the Breton Regionalist Union in 1898. He edited La Revue de Bretagne with the Count René de Laigue.
The publication appeared from 1902 to 1943. In 1909, deputed by the first district of Vannes, he led the delegation to lobby for the teaching of the Breton language in Breton schools and colleges. He attempted to interest the Minister of State for education Gaston Doumergue who refused their request by claiming that "teaching of Breton would support the separatist tendencies".
He participated in World War I as a volunteer, joining French forces at the age of 56. For his actions he was decorated with the Croix de guerre 1914-1918 and made Chevalier de la Légion d'honneur. After the war he withdrew from national politics, but continued to pursue his demands for official teaching of the Breton language.
In January 1919 he wrote a public declaration reasserting the demand published by Libre parole. He died at the château de Penhoët, Avessac.
In the 1920s he became a member of the Breton cultural movement Seiz Breur, sometimes writing under the Breton pseudonym Hoël Broërec'h.