After making his retreat at Mont-Saint-Michel, Louis Marie set off to find the missionary band headed by one of the greatest of Breton Missioners, Father Leuduger, and having caught up with them in Dinan, was accepted as a member of the team. Over the next few months he was involved in many missions in the dioceses of Saint-Malo and Saint-Brieuc, including one in his own birth-place, Montfort-sur-Meu, and others at Plumieux and La Chèze (where he rebuilt an ancient chapel, long since fallen into ruins, dedicated to Our Lady of Pity). Always he would choose for his own attention the poorest areas of the towns where the missions were held, and would often introduce some new initiatives for the relief of the poor, for example a soup-kitchen which was set up in Dinan.
He was perhaps not at his best, however, working with a team and, after several months, he left the mission band to spend a year at Saint-Lazare, just outside Montfort-sur-Meu, with two lay-brothers who had joined him. Here he occupied himself with teaching catechism to those who came to this ancient priory and schooling the two brothers in the art of community living. At the end of a year, he must have felt that other places offered him more opportunites for preaching missions and in 1708 he left to work in the Diocese of Nantes.
For two years, he preached many missions in and around Nantes, the vast majority of which proved extraordinarily successful in terms of the conversions wrought among the people. His reputation as a great missioner grew, but most of all he began to be known everywhere, by the ordinary people, as "the good Father from Montfort". He tried to perpetuate the spiritual results of his missions by setting up confraternities and associations which would encourage the people to be faithful to their renewal of Baptismal commitment, and by erecting physical reminders of the mission in the form of mission crosses. At Pontchateau, he attracted many thousands of people to help him in the erection of a more imposing reminder of the love of God, in the shape of a huge Calvary.
The Calvary of Pontchateau, however, was to be the cause of one of his greatest disappointments. On the very eve of its blessing, the Bishop, having heard that it was to be destroyed on the orders of the King himself, forbade its benediction. The whole sorry affair of the condemnation of the Calvary was the result of jealousy and petty revenge, but the Bishop evidently felt he had no choice but to curb the "excesses" of this extraordinary priest, and a few days later he forbade Louis Marie to do any more preaching in his diocese. This was just one, though perhaps the greatest, of the many instances where Louis Marie was called to share in the Cross of Christ. He did not let it get him down, but on the contrary reflected and meditated on it, and set down his reflections in one of his short writings, the Letter to the Friends of the Cross.
Although he was not banned from all work in the Diocese of Nantes, it was clear that if he wished to continue his preaching, he would have to go elsewhere. On the invitation of the Bishop of La Rochelle, he left Nantes in 1711 and entered the last period of his life, preaching missions in the Dioceses of La Rochelle and Luçon, in the Vendée region of France.