[2][3][4][5] Born in Saint-Denis on 2 July 1892, Mercier lived most of his life in the 16th arrondissement of Paris, barely 300 meters from the parish church of Saint-Honoré d'Eylau, where the Étoile des Deux Lacs was based.
[1] These qualities made him a versatile footballer, who was capable of "playing in all positions", mainly as a defender, but also at number 9; for instance, he even scored two goals in December 1910, thanks to his powerful volley shots.
[1] Mercier earned all of his three international caps for France within two months in April and May 1910, when he was not yet 18 years old, in friendly matches against Belgium, England amateurs, and Italy, all of which ending in losses with a total of 20 goals conceded.
[1][2][4][5] Nevertheless, the local press stated that "Mercier was excellent in every way" against the English,[8] and on his debut against the Belgians, the press stated that "the best full-back was undoubtedly Mercier, skillful and very fast",[1] an impressive compliment for a player aged only 17 years and 9 months, thus becoming the youngest-ever player of the French national team at the time, a record that has since been surpassed by fellow 17-year-olds Félix Vial (1911), Maurice Gastiger (1914), and Warren Zaire-Emery (2023).
[9] Two months later, in July 1910, he volunteered to military service for three years, but was nonetheless a substitute for France in a friendly against Luxembourg in October 1911.