Although Lagrange's family was Catholic, the young boy didn't show signs of remarkable piety as was seen in the late nineteenth century of those who would later become saints.

While studying medicine at Bordeaux, he experienced what he described as a religious conversion after reading Life, Science, and Art by the Breton writer Ernest Hello (1828–85).

He would later recall to his Dominican confrere, Rosaire Gagnebet: I was able to glimpse how the doctrine of the Catholic Church is the absolute Truth concerning God and his intimate life and concerning the human person, his origin and his supernatural destiny.

As fate would have it, it was at the Sorbonne in which Garrigou would meet the young Bergsonian – destined to become a major proponent of Thomism – Jacques Maritain where they would have a good relationship throughout the 1930s to 1940s.

In 1906, the deteriorating health of Etienne Hugueny caused him to resign from his post and Garrigou was called to ascend to the chair of dogmatic theology at Le Saulchoir.

As a subsequent result of this change, Garrigou commenced his approfondissement (in-depth study) in the works of Saint Thomas Aquinas and other Thomists who would guide him throughout his life.

[8] Father Garrigou-Lagrange, the leading proponent of "strict observance Thomism", attracted wider attention when in 1946 he wrote against the Nouvelle théologie theological movement, criticizing elements of it as Modernist.

[9] He is also said to be the drafter of Pope Pius XII's 1950 encyclical Humani generis, subtitled "Concerning Some False Opinions Threatening to Undermine the Foundations of Catholic Doctrine".

[10] In politics, like many neo-scholastic theologians of his time, Garrigou-Lagrange was a strong supporter of the far-right movement Action Française and he also sympathized with Vichy France.

In 1941 he praised the French collaborationist regime and its Chief of State Pétain in a letter written to his former disciple Jacques Maritain: "I am entirely with the Marshal, I see him as the Father of the patrie, blessed with a good sense verging on genius, and as a truly providential man".

[17] He is commonly held to have influenced the decision in 1942 to place the privately circulated book Une école de théologie: le Saulchoir (Étiolles 1937) by Marie-Dominique Chenu, O.P., on the Vatican's Index Librorum Prohibitorum as the culmination of a polemic within the Dominican Order between the Angelicum supporters of speculative scholasticism and the French revival Thomists who were more attentive to historical hermeneutics.