Désiré Mercier was born at the château du Castegier in Braine - l'Alleud, as the fifth of the seven children of Paul - Léon Mercier and his wife Anne - Marie Barbe Croquet.

Entering the minor seminary at Mechelen in 1861, he then attended Mechelen's Grand Seminary from 1870 to 1874.

Mercier received the clerical tonsure in 1871, temporarily served as dean of the seminary, and was finally ordained to the priesthood by Archbishop Giacomo Cattani, the nuncio to Belgium, on 4 April 1874.

He obtained his licentiate in theology (1877) and doctorate in philosophy from the University of Louvain, and also took courses in psychology in Paris.

Mercier was the nephew of Reverend Adrien Croquet. In the 1860s Rev. Croquet (renamed Crockett) became a missionary to the Grand Ronde Indian Reservation in Oregon. In the 1870s, a cousin to Mercier, Joseph Mercier, joined his uncle Rev. Croquet in Oregon and married into a Native American tribe. Today, there are several thousand descendants of Joseph as members of the tribe.

Three of Mercier's sisters became nuns, and his brother Léon became a physician.

In 1877 Mercier began teaching philosophy at Mechelen's minor seminary, of which he also became spiritual director. His comprehensive knowledge of Saint Thomas Aquinas earned him the newly - erected chair of Thomism at Louvain's Catholic university in 1882.

It was in this post, which he retained until 1905, that he forged a lifelong friendship with Dom Columba Marmion, an Irish Thomist. Raised to the rank of Monsignor on 6 May 1887, Mercier founded the Higher Institute of Philosophy at the Louvain University in 1899, which was to be a beacon of Neo - Thomist philosophy.

He founded in 1894 and edited until 1906 the Revue Néoscholastique, and wrote in a scholastic manner on metaphysics, philosophy, and psychology, several of his works being translated into English, German, Italian, Polish, and Spanish. His most important book was Les origines de la psychologie contemporaine (1897).

His reputation within his field obtaining the recognition of Pope Pius X, Mercier was appointed Archbishop of Mechelen and thus Primate of Belgium on 7 February 1906. He received his episcopal consecration on the following March 25 from Archbishop Antonio Vico, and took as his episcopal motto: Apostolus Jesu Christi.

Mercier was created Cardinal Priest of S. Pietro in Vincoli by Pope St. Pius X in the consistory of 15 April 1907.

Pope Benedict XV sent his portrait and a letter of whole - hearted support to Mercier in 1916, and at one point told him "You saved the Church!"

He was also one of the cardinal electors in the 1922 papal conclave, which selected Pope Pius XI.

Mercier suffered from persistent dyspepsia, and in early January 1926 he underwent surgery for a lesion of the stomach. During surgery, the anaesthetized Cardinal even held a conversation with his surgeon.

In his final days, Mercier was visited by the likes of King Albert and Queen Elizabeth, Lord Halifax, and family members. He entered a deep coma around 2:00 p.m. on January 23, and died an hour later, at age 74. The Cardinal was buried at St. Rumbolds Cathedral.

The Cardinal harbored great devotion to the Sacred Heart.

Mercier is also known for opposing the use of Dutch (language spoken by about two third of the Belgian population). This was sparked by a conversation with a Flemish priest, whom he told the following: "Moi je suis d'une race destinée à dominer et vous d'une race destinée a servir" (English: "I belong to a race destined to dominate and you belong to a race destined to serve").

Also worthy of note, is Mercier's role in recognizing the mathematical talent of a young Belgian seminarian, Georges Lemaître, and urging Lemaitre to study Einstein's theories of relativity. Lemaître became an early expert in general relativity as it applied to cosmological questions, and he went on to propose an expanding model of the universe, based on both Einstein's and de Sitter's models. His Primeval Atom hypothesis was developed by abbé Georges Lemaître himself, university of Louvain, and Gamow, Alpher and Herman into the better known Big Bang theory of the origin of the universe.

In 1914 the German army attempted a surprise invasion of France by invading neutral Belgium. Mercier had to leave his see on August 20 of that same year to attend the funeral of the late Pius X, and participate in the following conclave. His first port of call upon his return to Belgium was Havre to meet wounded Belgian, French and British troops. When he returned to his archdiocese, the Mechelen Cathedral was partially destroyed. In the Imperial German atrocities that ensued, thirteen of the priests in Mercier's diocese were killed, not to mention many civilians, by Christmas 1914. At that time, when Mercier's pastoral, Patriotism and Endurance, was distributed to be read aloud in all Belgian churches in January 1915. The pastoral letter had to be distributed by hand as the Germans had cut off the postal service. His passionate, unflinching words were taken to heart by the suffering Belgians. He embodied Belgian resistance to the occupying power. He sometimes became a focus of Allied Propaganda during the War. He was kept under house arrest by the Germans, and many priests who had read the letter aloud in public were arrested as well.

Following World War I, Mercier undertook an excursion to raise funds to rebuild and stock a new library of the University of Leuven. The original library had been burned by the Germans in the war. In his travels to raise funds, Mercier visited New York for his first and only time.

From 1921 to 1926 he held regular conversations with Anglican theologians, notably Edward, Lord Irwin (later the ecumenical pioneer Lord Halifax), foreshadowing the Church's future dialogue with the Anglicans. Anglicanism, Mercier believed, must be "united, not absorbed".