Julien Maunoir was born on October 1, 1606 at Saint-Georges-de-Reintembault in Brittany to a Breton peasant family. In 1625, Julien entered the Society of Jesus at Paris.
Ordained priest in 1637, Julien undertook the vocation of the Breton-language missionary — learning the Breton Celtic language (the ancient Celtic language of Brittany, distinct from French) and preaching to the Breton peasantry in the Breton vernacular.
For the 43 years (1640–1683), Julien travelled across the Brittany countryside conducting 'missions' — the 17th-century Jesuit intensive-preaching missions that lasted 3-4 weeks in each parish, producing the systematic Catholic revival of the Breton peasant community.
Julien conducted documented 439 missions across Brittany — the most extensive Jesuit mission-preaching campaign in the French Catholic tradition. Julien personally composed Breton-language catechisms, hymns, and spiritual guides — producing the Breton-language Catholic literary corpus.
He died at Plévin on January 28, 1683 at the age of 76.