Amerigo Vespucci, trained for a business career, is the third son of Ser Nastagio (Anastasio), a Florentine notary, and Lisabetta Mini.

Amerigo had been educated by his uncle, Fra Giorgio Antonio Vespucci, a Dominican friar of the monastery of San Marco in Florence.

While his elder brothers had been sent to the University of Pisa to pursue scholarly careers, Amerigo Vespucci had embraced a mercantile life, and had been hired as a clerk by the Florentine commercial house of Medici, headed by Lorenzo de' Medici.

Vespucci had acquired the favor and protection of Lorenzo di Pierfrancesco de' Medici, who had become the head of the business after the elder Lorenzo's death in 1492.

In March of that year, the Medici dispatched the thirty-eight-year-old Vespucci and Donato Niccolini as confidential agents to look into the Medici branch office in Cádiz, whose managers and dealings were under suspicion.

In April 1495, by the intrigues of Bishop Juan Rodríguez de Fonseca, the Crown of Castile had broken their monopoly deal with Christopher Columbus and had begun handing out licenses to other navigators for the West Indies.

Just around this time, Vespucci, engaged as the executor of Giannotto Berardi, an Italian merchant who had recently died in Seville, had organized the fulfillment of Berardi's outstanding contract with the Castilian crown to provide twelve vessels for the Indies.

After these were delivered, Vespucci had continued as a provision contractor for Indies expeditions, and is known to have secured beef supplies for at least one (if not two) of Columbus' voyages.

A letter published in 1504 purports to be an account by Vespucci, written to Florentine statesman Piero Soderini, of a lengthy visit to the New World, leaving Spain in May 1497 and returning in October 1498.

However, modern scholars have doubted that this voyage took place, and consider this letter a forgery.

Whoever did write the letter makes several observations of native customs, including use of hammocks and sweat lodges.

The names of Amerigo Vespucci's ships were the San Antiago, Repertaga, Wegiz, and Girmand.

About 1499–1500, Vespucci joins an expedition in the service of Spain, with Alonso de Ojeda (or Hojeda) as the fleet commander.

The intention is to sail around the southern end of the African mainland into the Indian Ocean.

Three unpublished familiar letters from Amerigo Vespucci to Lorenzo de' Medici will be rediscovered in the eighteenth century.

One describes a voyage made in 1499–1500 which corresponds with the second of the "four voyages".

The two disputed letters claim that Vespucci made four voyages to America, while at most two can be verified from other sources.

At the moment there is a dispute between historians on when Vespucci visited the mainland the first time.

Some historians like Germán Arciniegas and Gabriel Camargo Pérez think that his first voyage was made in June 1497 with the Spanish Pilot Juan de la Cosa.

After Ojeda and Vespucci, bound for India by way of the Cape of Good Hope in 1499, hit land at the coast of what is now Guyana, the two seem to have separated.

Vespucci sails southward, discovering the mouth of the Amazon River and reaching 6°S, before turning around and seeing Trinidad and the Orinoco River and returning to Spain by way of Hispaniola.

The letter to Lorenzo di Pierfrancesco de' Medici, claims that Vespucci determined his longitude celestially on August 23, 1499, while on this voyage.

However, this claim may be fraudulent, which could cast doubt on the letter's credibility.