The term King's Men referred to a party of the Númenóreans, composed of the majority of the people living on Númenor in the later half of the Second Age, who followed the policies of Tar-Ancalimon and his successors to build the pride and wealth of the isle. The opposing party, the Faithful, were the minority group and lived mostly in the west of Númenor. The King's Men received this name because they had the support of most of the Númenórean Kings after Tar-Atanamir, who led the people to a policy of rebellion against the Valar. The King's Men became hostile to the Eldar and the Valar because they envied their immortality and despised the Ban of the Valar. To compensate for the restriction the Ban placed on their voyaging, they sailed east from Númenor to Middle-earth, where they set up great dominions and colonies in the south among the Haradrim and laid heavy tribute on them. In this way they amassed much wealth, which they brought back to Númenor.
Because the Shadow lay heavy upon their hearts, the King's Men greatly feared Death and "sought to delay it by all means that they could". These attempts to escape their fate actually brought death upon them sooner, and the lifespans of their Kings waned. They filled the land of Númenor with great tombs to preserve the memory of their dead. In the meantime, their everyday lives became ever more decadent: to ease their fears, they "drank and they feasted and they clad themselves in silver and gold".
Though the King's Men had enjoyed generations of favor from the Kings of Númenor, and no doubt helped in oppressing the Faithful as the Kings grew more suspicious of them, eventually a supporter of the Faithful took the Sceptre once again. This King's name was Tar-Palantir. He wished to return Númenor to a state of friendship with the West, but the majority of the Númenóreans still clung to the policies of the King's Men. Tar-Palantir's younger brother, Gimilkhâd, sided with his rebellious fathers, however. He took up the leadership of the King's Men and stood in opposition to his brother, but generally kept his doings secret.
It was probably to a few lords of the King's Men that Sauron came to deliver some of the Nine Rings of Power, for three of the Nazgûl were said to be "lords of Númenórean race". Most of the King's Men living in Middle-earth Sauron and his servants assailed, however. Eventually, the last King of Númenor, Ar-Pharazôn the son of Gimilkhâd, resolved to defeat Sauron in retaliation for this aggression, and he and his men sailed to Middle-earth and captured the Maia.
Once in Númenor, Sauron completed the corruption of the King's Men with ease, inciting them to worship Melkor and use the few Faithful still living in the land as sacrifices. They also brutally oppressed the Men of Middle-earth during this period. The corrupting power of Sauron affected their minds deeply, so that they became "quick to anger", and hated all those in the land more fortunate than them. Nor did they escape death, for all sorts of sicknesses and mental afflictions assailed them, to the point that they "cursed themselves in their agony". When the Valar sent messages of warning in the form of eagle-shaped clouds to them, they fell upon their faces in great fear, and some repented of their actions for a while. In the end, however, Sauron's mastery over them was so complete that he could walk about the land "setting man against man", and cause them to slay each other. At last, when Ar-Pharazôn had grown old and was in great fear of death, Sauron persuaded him to take a great army to Aman and challenge the Valar for immortality. When he did this, his army foundered in the Sea and Númenor was also destroyed.
Most of the King's Men thus perished in the destruction of Ar-Pharazôn's fleet and the Downfall of Númenor, but those who had taken up residence in Middle-earth survived. When Sauron returned to Mordor, they entered into his service once again, and became known as the Black Númenóreans.