Merion was the younger son of King Constantine Corneu of Dumnonia, and he inherited the western-most portion of his kingdom, Kernow, now Cornwall, under the overlordship of his brother, Erbin. He was succeeded by his more famous son, Marc.

The name Merion is recorded in a single king list dating from the 16th century, although his placement lines up with 11th century folkloric records. He is generally considered legendary.