Cemendur was descended from a mighty line of lords: his great-grandfather was Elendil himself, and he was the grandson of Anárion and son of Meneldil. Nonetheless, of his reign as King of Gondor we know almost nothing. It seems that Gondor had peace during his reign, since no significant historical event is chronicled for that period.
Cemendur ruled in Gondor for eighty-six years, and was succeeded by his son Eärendil.
The date of Cemendur's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
Cemendur's name appears to contain kemen-, relating to the earth (as for example in Kementári, 'Queen of the Earth'). This is part of a sequence: the early Kings of Gondor had names that devoted them to cosmic and elemental forces. From Cemendur's great-grandfather Elendil to his grandson Anardil, the Kings' names devoted them in turn to the stars, the Moon, the heavens, the Earth, the Sea and the Sun.