He started his working life in a bank owned by a cousin as well trading in refined and dyed wool. Due to his efforts he became one of Italy's wealthiest bankers. By supplying money to his friend Baldassare Cossa, Cossa was probably able to buy his cardinal's hat, subsequently becoming Pope John XXIII (1410-1415), while Giovanni became the papal banker.
Even though the Medici bank had branches in Rome, Venice, Genoa, Naples and Gaeta as well as in his native Florence, over half of the profits came from Rome. 
Giovanni supported Pope John XXIII against King Ladislas of Naples and, when Pope John XXIII was deposed and imprisoned, he remained loyal and secured his release. In 1417 Oddone Colonna became Pope Martin V and, even though losing his monopoly, Giovanni still did business with and for the papacy. 
He was honest and understanding without seeking political office, and he used his money to his own advantage as well as to assist the people of Florence when they had to pay their taxes.

Source: Leo van de Pas