He was born at Paris, entered the Society of Jesus at the age of sixteen, and, after a distinguished course of study, taught at first the humanities, then philosophy, at Clermont-Ferrand (1643–1653), and theology at Bourges (1653–1681).

[citation needed] In 1618, he published for the first time the Libellus fidei, sent to the Holy See during the Pelagian controversy by Julian, Bishop of Eclanum in Apulia.

In 1655, he wrote Regulae fidei catholicae de gratia Dei per Jesum Christum, and published the work at Bourges.

Much praise is bestowed on Garnier by later learned writers for the great amount of historical knowledge displayed in his dissertations, but he is also severely blamed for his arbitrary arrangement of the writings of Mercator and for his criticism of the original.

[1] Garnier edited in 1675 at Paris the Breviarum causae Nestorianorum (composed before 566 by Liberatus, an archdeacon of Carthage), correcting many mistakes and adding notes and a dissertation on the Fifth General Council.