The island was brought up from the sea as a gift from the Valar to the Edain, the Fathers of Men who had stood with the Elves of Beleriand against Morgoth in the War of the Jewels. Númenor was meant to be a "rest after the war" for the Edain. Early in the Second Age the greater part of those Edain that survived their defeat from Morgoth journeyed to the isle, sailing in ships provided and steered by the Elves. The migration took fifty years and brought 5,000 to 10,000 men, women and children.
The realm was officially established in S.A. 32, and Elros Half-elven, son of Eärendil, and brother of Elrond and descendant of all the royal houses of Elves and Edain, became the first King of Númenor. Under his rule, and those of his descendants, the Númenóreans rose to become a powerful people. The first ships sailed from Númenor to Middle-earth in the year 600 of the Second Age.
The Númenóreans were forbidden by the Valar from sailing so far westward that Númenor was no longer visible, for fear that they would come upon the Undying Lands, to which Men could not come. For a long time, Númenor remained friendly with the Elves, both of Eressëa and of Middle-earth, and between S.A. 1693-1700, they assisted Gil-galad in the War of the Elves and Sauron, which broke out after the forging of the Great Rings, in particular the One Ring. King Tar-Minastir and the forces of Númenor were without peer in war, and together with the Elves, they were able to temporarily defeat Sauron. Over time the Númenóreans became jealous of the Elves for their immortality, and began to resent the Ban of the Valar and to rebel against their authority, seeking the everlasting life that they believed was begrudged them. They tried to compensate for this by going eastward and colonizing large parts of Middle-earth, first in a friendly manner, but later as cruel tyrants. Soon the Númenóreans came to rule a great coastal empire that had no rival. Few (the "Faithful") remained loyal to the Valar and friendly to the Elves.
In the year 3255 of the Second Age, the 25th king, Ar-Pharazôn, sailed to Middle-earth and landed at Umbar. Seeing the might of Númenor, Sauron's armies fled and Sauron surrendered without a fight. He was brought back to Númenor as a prisoner but he soon became an advisor to the king and promised the Númenóreans eternal life if they worshipped Melkor. With Sauron as his advisor, Ar-Pharazôn had a 500-foot (150 m) tall temple to Melkor erected, in which he offered human sacrifices to Melkor (those selected to be sacrificed were Elendili, Númenóreans who were still faithful to the Elves).
During this time, the White Tree Nimloth, which stood before the King's House in Armenelos and whose fate was said to be tied to the line of kings, was chopped down and burned as a sacrifice to Melkor at Sauron's direction. Isildur, heroically and at great personal risk, rescued a fruit of the tree which became an ancestor of the White Tree of Gondor, preserving the ancient line of trees.
Prompted by Sauron and fearing old age and death, Ar-Pharazôn built a great armada and set sail into the West to make war upon the Valar and seize the Undying Lands, and by so doing achieve immortality. Sauron remained behind. This force was quoted by Tolkien as the 'greatest force ever assembled on Arda'. In the year 3319 of the Second Age, Ar-Pharazôn landed on the shores of Aman. As the Valar were forbidden to take direct action against Men, Manwë, chief of the Valar, called upon Eru. The Undying Lands were removed from the world forever, and the formerly flat Earth was made into a globe. Númenor was overwhelmed in the cataclysm and sank beneath the sea, killing its inhabitants, including the body of Sauron who was thereby robbed of his ability to assume fair and charming forms, forever appearing in the form of a Dark Lord thereafter.
Elendil, son of the leader of the Faithful during the reign of Ar-Pharazôn, his sons and his followers had foreseen the disaster that was to befall Númenor, and they had set sail in nine ships before the island fell. They landed in Middle-earth and founded the kingdoms of Arnor and Gondor.