Gil-galad was the last High King of the Noldor in Middle-earth. In early texts, he was the son of Fingon the Valiant, son of Fingolfin, son of Finwë, first High King of the Noldor. Other sources and versions of the text however, indicate Gil-galad is the Son of Orodreth of the House of Finarfin. This would make him grandson of Angrod, the brother of Finrod Felagund and Galadriel. It is through this descent that his High-kingship of the Noldor is explained.
The early history of Gil-galad is somewhat obscure, as Tolkien himself repeatedly changed his mind about who exactly Gil-galad was, and how to fit him into the already existing history of the First Age (see concept and creation, below). Gil-galad was certainly born in Beleriand at some point during the First Age. He was perhaps at some point sent to live in the Falas with Círdan the Shipwright, but this is unclear — certainly after the fall of Nargothrond he was living on the Isle of Balar with Círdan, and became the last High King of the Noldor following the fall of Gondolin and the death of Turgon. Gil-galad, despite his long life (he was born during the First Age and lived through almost the entirety of the Second), was not known to have a wife, nor any children. Due to this, and the fact that there were few Noldor remaining in Middle-earth, the title of High King of the Noldor in Exile ended with his death.
After the War of Wrath and the end of the First Age, Gil-galad founded a realm in the coastal region of Lindon along the shores of Belegaer, the Great Sea. At its height, his realm extended eastward as far as the Misty Mountains, though most of the Eldar remained in Lindon and in Elrond's refuge of Rivendell.
According to The Fellowship of the Ring, Gil-galad was the first of the Eldar to mistrust a stranger calling himself Annatar, and forbade him from entering Lindon. His mistrust was well founded, for it was soon learned that Annatar was in fact Sauron. After Sauron forged the One Ring, Gil-galad was given one of the Three Elven Rings: Vilya, the Ring of Air (and most probably also Narya, the Ring of Fire). Just before Gil-galad's death, Elrond was given Vilya for safekeeping (and Narya was given to Círdan).
During most of the Second Age, Gil-galad enjoyed the friendship of the Númenóreans. This proved very useful as during the War of the Elves and Sauron; a great Númenórean force under the command of their king Tar-Minastir helped Gil-galad destroy Sauron's armies.
After the Downfall of Númenor and the establishment by the Elendili of the Dúnadan kingdoms in exile, there was peace in Middle-earth. In the Age's closing years, however, Sauron reappeared with a newly formed army and a war against the kingdom of Gondor, closest to his old home of Mordor. Gil-galad then formed the Last Alliance of Elves and Men with Elendil, High King of the Dúnedain-in-Exile, The armies of Elves and Men, victorious after the Battle of Dagorlad, laid siege to Sauron in Mordor.At the end of the siege, both Gil-galad and Elendil aided in the overthrow of Sauron's physical body yet perished themselves in the assault. At the Council of Elrond, Elrond says that only three people survived the final battle with Sauron: Isildur, Círdan, and himself.
A record left by Isildur in Minas Tirith implies that Sauron himself slew Gil-galad, with the heat of his bare hands.
Gil-galad's spear was named Aiglos or Aeglos, meaning "snow-point" or "snow-thorn" or more commonly "icicle" (aeg: sharp, pointed; los: snow) because when orcs saw his spear, they would recognize it by its reputation to bring a cold death to them. Elrond said that at the battle of Dagorlad, "we had the mastery: for the Spear of Gil-galad and the Sword of Elendil, Aiglos and Narsil, none could withstand."