In the times of the last king of Númenor, Ar-Pharazôn, before the capture and imprisonment of Sauron, the king's most trusted councillor and friend was Amandil, Lord of Andúnië. After the king had brought the defeated Sauron to Númenor, Sauron slowly corrupted the king with flattery and praise. Sauron seeing Amandil as his chief rival for the ear of the king spoke out against him. Amandil seeing that he was losing his battle with Sauron withdrew to Rómenna with all that were left of the Faithful. Where he took council with his son Elendil, and his grandsons Isildur and Anárion.
Now Nimloth was a tree of very high lineage. Of old Yavanna gave as a gift Galathilion, the Tree of Túna; that was in the image of Telperion, the Silver Tree of Valinor, to the Eldar. A seedling of Galathilion was Celeborn the White Tree that grew in the midst of Eressëa. Nimloth was a seedling from Celeborn. Nimloth blossomed and grew at the Court of the King at Armenelos.
King Tar-Palantir, had before prophesied saying that when the Tree perished the line of the King would come to an end.
Sauron was urging the king to cut down Nimloth, the White Tree of Númenor.
Isildur hearing of this and remembering the words of Tar-Palantir, took his own council stole into Armenelos to the court of the king, where the tree was. Isildur in disguise eluded the guards and stole the one last fruit from the tree. At this point he was discovered and received many cuts and injuries, but he escaped and brought the fruit to Amandil. Amandil planted the seed from the fruit and the next spring a sapling sprouted, when this happened Isildur, who had lain near death through the winter, awoke and was never bothered by his injuries again.
Soon after that Sauron had the tree cut down.
Later as the fleets of Númenor sailed into the west. The Faithful were onboard their ships ready to go east. Isildur had the young sapling on his ship. After when the Faithful established themselves in Middle-earth the sapling was replanted in MinasIthil, before the House of Isildur.