Louis Marie Grignion was born 31 January 1673 in the small town of Montfort-sur-Meu, just West of Rennes in Brittany, France. He was the eldest surviving child of the large family of Jean-Baptiste Grignion and his wife Jeanne Robert.
Louis Marie passed most of his infancy and early childhood in Iffendic, a few miles from Montfort, where his father had bought a farm known as "Le Bois Marquer". According to those who knew him at this early stage, he showed signs even then of a spiritual maturity uncommon in one of his age.
At the age of 12, he entered the Jesuit College of St Thomas Becket in Rennes, where, as well as doing well in his studies, he developed some of the enthusiasms which were to mark his later life. Listening to the stories of a local priest, the Abbé Julien Bellier, about his life as an itinerant missionary, he was fired with zeal to preach missions. And, under the guidance of some other priests he began to develop his strong devotion to the Blessed Virgin. At the same time, he began to experience the deprivations suffered by the very poor, and his love and care for them grew, not only in theory but in a practical way.
At some time during his college days, he became aware of a call to the priesthood, and at the end of his ordinary schooling, began his studies of philosophy and theology, still at St Thomas in Rennes. However, he was given the opportunity, through a benefactor, to go to Paris to study at the renowned Seminary of Saint-Sulpice. He set out for Paris towards the end of 1693.