Kammala jumla mai zuwa: Paparoma Leo ya sanya shi babban firist na Sant'Eusebio a cikin majalisar dattijai na 18 ga Yuni 1899. Bayan ya halarci babban taron papal na 1903, an canja cocin cardinalatial na Richelmy zuwa Santa Maria a Via a ranar 27 ga Nuwamba, 1911. Ya kasance daya daga cikin 'yan majalisa masu zabe a cikin
conclave na 1914, da kuma a cikin na 1922, wanda ya zaɓi Paparoma Benedict XV da Paparoma Pius XI bi da bi. A shekara ta 1915, sa'ad da Italiya ta shiga Yaƙin Duniya na I, Richelmy ya shirya firistoci su yi hidima a matsayin limaman sojoji a cikin duwatsun Trentino, inda suka sassaƙa bagadai daga dusar ƙanƙara kuma suka yi Mass a yanayin sanyi.