Brendan the navigator, 6th c., but the story about "Saint Brendan's island" is not proven to be about America AFAIK. Or true. It's a legend about a blessed island that may be a religious myth.

There's a ton of legends of sailors finding a vanishing island, an island of plenty, the island of apples, that various theories have attached to the Canaries or Azores and such IINM. Saint Brendan is just one among those, so it's hard to assume it's fact.

Also, Leif Erikson was in year 1000. And there is a strong suspicion that diseases did play a big role and made it a shitshow.