The eldest son of Elendil, born in Númenor around 3209 of the Second Age. He was brother to Anárion and father to Elendur, Aratan, Ciryon and Valandil of Arnor.

He was among the Faithful (Elendili) of Númenor. When he heard that the White Tree of Númenor, Nimloth, might soon be cut down, he disguised himself and snuck into the courts of the king. This area was forbidden to the Faithful, but he was able to get in and take a fruit from the tree. The guards were roused when he tried to leave, and he had to fight his way out. They did not recognize him, and he got away wounded. He was near death when he got back, but when much later the first leaf opened on the shoot that grew from the plant, he was cured.

Isildur escaped the Drowning of Númenor. He and his brother then founded the realm of Gondor. Isildur lived in Minas Ithil, and the White Tree was planted outside of his house.

When Sauron attacked Gondor, he took Minas Ithil and destroyed the White Tree. Isildur escaped, and took with him a seedling of the Tree.

This was the attack that sparked the Last Alliance. Isildur fought, and saw Sauron kill his father. To avenge his father's death, he picked up what was left of Elendil's sword, Narsil, and cut the One Ring from Sauron's hand. He then claimed it for himself. He would not listen to counsel from Elrond and Círdan, and kept the One Ring for an heirloom of his house.

He returned to Minas Anor and planted the seedling of the White Tree there in memory of his brother. Then he started to travel to his father's realm in Eriador.

At the Gladden Fields his company was overwhelmed by orcs, and many were killed, including his three oldest sons. He escaped by using the ring to make himself invisible, but when he was swimming the River Anduin, it betrayed him and slipped off. He was killed by orc arrows piercing his heart and throat.