He studied philosophy at Louisiana Fleche and later assigned to the college at Saint-Ives at Quimper, Brittany where he taught Latin and Greek.
He worked hard and within two months he was sufficiently fluent to be able to preach in Breton. Maunoir is considered a noted orthographer of the Breton language, having completed a Breton grammar. He continued to preach in the hamlets of Brittany until he went to Tours to begin his theological studies prior to ordination.
He was ordained in 1637 and returned to Brittany in 1640 and was assigned again to Quimper.
He was found to be uniquely suited for the difficult task of evangelizing the impoverished people of Brittany. Father Maunoir worked as a missionary to the Breton people for 43 years, and managed to give a Christian meaning to what had become pious customs.
By 1683 had formed almost 1,000 Breton missionaries who carried on the pastoral works that he had begun. Julien Maunoir died in Plévin, Brittany, on 28 January 1683 and was buried in the parish church grounds at the insistence of his people.
He was beatified by Pope Pius XII in 1951.