He was the eldest surviving child in a large family of eighteen children born to Jean-Baptiste Grignion and Jeanne Robert. His early years were spent primarily in Iffendic, where his father had acquired a farm named “Le Bois Marquer.”

This rural upbringing played a significant role in shaping his character and spiritual outlook.

From a young age, Louis exhibited signs of exceptional spiritual maturity. At the age of 12, he entered the Jesuit College of St. Thomas Becket in Rennes. Here, he excelled academically and developed a deep enthusiasm for spirituality and missionary work. Influenced by stories from local priest Abbé Julien Bellier about life as an itinerant missionary, he felt inspired to preach missions among the poor.

During his time at college, Louis began to feel a calling to the priesthood. After completing his ordinary schooling, he continued his studies in philosophy and theology at St. Thomas in Rennes.

Louis’s health suffered due to the harsh conditions of his living situation combined with self-imposed penances; after less than two years, he fell seriously ill and required hospitalization at Hôtel-Dieu. Remarkably, he survived this illness despite rigorous treatments common at that time. Upon recovery, he found himself with an opportunity to enter Little Saint-Sulpice in July 1695, where he served as librarian and immersed himself in studying spirituality—particularly focusing on Mary’s role within it.