Most ancient civilizations thought the world was flat for a long time, but the Greeks knew by Pythagoras in the 6th century and India knew by the Gupta era. Most people knew in Columbus' day too (you can tell several ways, the easiest is watching a ship sail over the horizon. You'll notice the ship disappears from bottom first not in equal proportion). The myth of this is created by an author named Washington Irving who had written a popular biography of Columbus in the 19th century. He also created the Legend of Sleepy Hollow and the myth of Washington cutting down the cherry tree (in another faked biography).

This has been discussed many times, such as here and here, which discusses the origins of this myth. Quoting /u/Enrico_dandolo

Nobody in Europe thought the earth was flat. That was an anti-Catholic myth from much later. The Greeks knew it was a sphere based on the shadow cast on the Moon and later people continued to understand this. The globular Earth is referenced in the first book of Ovid (widely read in the Middle Ages). Adelard of Bath's (1080-1152) Questions on Nature even questions the nature of gravity (although he didn't know of the force itself, but rather the nature of the power holding us to this earth) by questioning what would happen if the spherical earth had holes in it like cheese?