After serving in the papal army and diplomatic service and as nuncio at the French court (1634-36), he entered the service of France and made himself valuable to King Louis XIII's chief minister, Cardinal Richelieu, who brought him into the council of state.
Ever as deft at the gaming table as with diplomacy, one evening his winnings were so great that a crowd gathered to see the stacks of gold écus, attracting the attention of the Queen; in her presence, Mazarin risked all, and won.