He took part in the creation of the Breton Separatist Party (PAB) at the first congress of Breiz Atao, held in September 1927 at Rosporden.

In 1934, he joined the Breton Federalist Movement, with Y. Gestalen, Francis Bayer of Kern, Goulven Mazéas and Rafig Tullou.

With other members of the Breton Federalist Movement, he founded the neo-Pagan "Kredenn Geltiek Hollvedel" (World Celtic Creed) group, of which he was the first arch-Druid.

However his group Kredenn Geltiek Hollvedel published a Druidic journal, Nemeton, one of the goals of which was to denounce Catholic influence in Brittany in the name of a supposed Aryan (Indo-European) cultural fraternity which bound a Nordic "New Europe" to the Celtic fatherland in Germany.

Morvan Marchal was buried in Paris in 1963 and re-buried in the family grave in Châteaugiron, near Rennes in Brittany on 24 January 1997.