Member of an old, no longer prosperous Florentine merchant family, Vespucci joined the trading firm of Lorenzo di Pierfrancesco de' Medici and was sent to Seville in 1494 to help in their agency there, becoming its head in 1495. He was involved in the fitting-out for Spain of Columbus's third expedition and, when news arrived late in 1498 of his discovery of part of the South American coast off Trinidad, Vespucci found a place on the expedition sent under Alonso de Hojeda to explore the coast more fully. Off Trinidad Hojeda broke off to continue westwards and Vespucci turned to follow the coast to the east. He was by then pilot and effective commander.

Upon what previous experience this promotion was based is not known. Probably he was a shareholder in the expedition; this could give a presumptive right to a command. Seville, with its community of cartographers and its coming and going of oceanic vessels, was a perfect observation post for the amateur student of navigation. But given the success of this voyage of 1499, which made the first European contact with the mouth of the Amazon and Brazil, and the astonishing determination and address which took him on his second voyage of 1501 (this time for King Manuel of Portugal), from Brazil at about latitude 5 degrees south to almost as far south as the Straits of Magellan, his biographers have been tempted to postulate some previous experience at sea. This was his last known voyage. Manuel, realizing that the coast explored by Vespucci tended south-west into the zone reserved for Spain by the 1494 Treaty of Tordesillas, no longer needed his services and he returned to Spain, where in 1505 he was given the important advisory post of pilot major.

No journals survive. Three quite brief letters describing his voyage to Pierfrancesco de' Medici remained in manuscript. Garbled and misleadingly elaborated versions were, however, published in 1504 (the New World and Letter of Amerigo Vespucci). Drawing in part on these, Martin Wadseemüller showed the Americas as a separate continent in his world map of 1507, naming Vespucci in its title and adding a note suggesting that the newly established land mass should be called America after him. The success of this suggestion, which gave another man's name to a continent first explored and reported on by Columbus, has led to a vast literature of belittlement and controversy around the very few known facts of Vespucci's life.

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