[1][2] Charost served as director of the Internship of Notre Dame de la Couture in Le Mans from 1892 to 1894, whence he became secretary to Archbishop Guillaume Labouré.

He was later made titular canon of the cathedral chapter of Rennes in 1899, and vicar general and director of secondary studies of the same in 1909.

During World War I, he encouraged the Catholics under his jurisdiction to practice passive resistance against the German Empire, who had occupied Lille;[3] the bishop also protested the Germans' actions to Cardinal Felix von Hartmann.

Pope Pius XI created him cardinal priest of S. Maria della Vittoria in the consistory of 11 December 1922.

Charost served as papal legate to the centennial celebrations in honor of Cardinal Charles Lavigerie in Algiers on 25 August 1925, to the fiftieth anniversary of the Université catholique de Lille on 14 March 1927, and to the celebrations in honor of St. Thérèse de Lisieux in Lisieux on 15 September 1929.