Prithu, son of Vena is a partial incarnation of Lord Vishnu who ruled the Earth for a long time. He is also mentioned in the Vedas. He is the seer of Mandala 10 hymn 148 of the Rigveda. He is famous for the legend of chasing the Earth Goddess. The Earth is named as Prithvi by his name.
According to Wikipedia, he is the first true sovereign because he there was no vegetation, agriculture and cities, grains etc., All these are developed during his reign. This is also confirmed by a Padma Purana passage which also says he ruled entire planet.
It is said that he started the rule during the secondary creation. But when did his reign end?