The first of ten nephews elected to the cardinalship by Rodrigo Borgia, Juan sought out a privileged position in the Borgia court. He helped Cesare negotiate an alliance with the Baron de Valois in 1499, impressing the young Captain General with his knowledge of French taste.
In an incident recorded by Bouchard, the Papal Master of Ceremonies, Juan and Cesare were seated, discussing financial matters, when a steward brought in a glass of wine. Juan took a sip, declared the wine to be false, accusing the astonished steward of drinking the real bottle. He then threw the wine over the steward and lit him on fire. Fortunately, the wine was not capable of igniting, but, unfortunately, the steward was still executed once the small fire was put out.
Such incidents must have impressed Cesare, as he shunned Rodrigo Borgia's banker Agostino Chigi, to invest with Juan.
Juan was gradually put in charge of Rome's finances. He used large amounts of the city's tax money to throw lavish parties for his friends, at one point tossing a hundred gold plates into the Tiber after their first use, outdoing Chigi, who had done the same with mere silver. Rome's citizens were also invited to the public areas of these parties, but many still complained privately that their money could be put to much better use.