After the War of Wrath the Edain left Middle-earth, but many Men remained there in the lands east and south of the Blue Mountains. The early centuries of the Second Age were a time of fear and darkness for the Men of Middle-earth, but after six hundred years the first vessel of the Númenóreans was seen off the coasts. While the Dark Years were passing in the lands east of the Sea, the civilization of Númenor had reached great heights, and soon the tall and mighty mariners of the Dúnedain began to explore Middle-earth. They brought gifts and knowledge to their fellow Men, who in turn came to know the Númenóreans as the 'Sea-kings', and some even considered them to be gods.

As time passed and the shadow fell on Númenor, the mariners began to exploit the lands of Middle-earth, building colonies and felling the forests (and according to some accounts even taking slaves back across the Sea). After the Downfall of Númenor, a few survivors led by Elendil landed in Middle-earth and built great works by means forgotten in later centuries. Even at the end of the Third Age, these monuments, especially Orthanc and Minas Tirith, were still referred to as the work of the Sea-kings.

The term is also used once in a fully capitalised form, in a reference to the 'Sceptre of the Sea-Kings' (the symbol of royalty in Númenor) in the Akallabêth. This capitalised usage is ambiguous: it may mean no more than the 'Sceptre of the Númenóreans', or it may be a more specific reference applying only to the Kings of Númenor.