Pope Leo created him cardinal priest of Sant'Eusebio in the consistory of 18 June 1899. After participating in the 1903 papal conclave, Richelmy's cardinalatial church was transferred to Santa Maria in Via on 27 November 1911. He was one of the cardinal electors in the conclave of 1914, and also in that of 1922, which selected Pope Benedict XV and Pope Pius XI respectively. In 1915, when Italy entered World War I, Richelmy organized priests for duty as army chaplains in the mountains of Trentino, where they carved altars out of snow and said Mass in below-zero temperatures.