Valacar was the son and heir of the great King of Gondor, Rómendacil II. While Rómendacil still sat on Gondor's throne, he developed a strong alliance with the Northmen who lived to the east of Mirkwood. To further the close friendship between Gondor and these Northmen, Rómendacil sent Valacar as an ambassador to the court of their leader Vidugavia, who styled himself King of Rhovanion.
Rómendacil had intended that his son should learn something of the language and culture of the Men of Rhovanion, but he had not foreseen how strongly Valacar would be drawn to the Northmen. He went so far as to wed Vidumavi, daughter of Vidugavia, a union that would bring great troubles in the future.
The Northmen did not share the long lives of the Gondorians, and Vidumavi died even before Valacar had succeeded his father as King. Nonetheless, she bore Valacar an heir. He was raised with the name Vinitharya in the eastern lands, but returned to Gondor with his father as Eldacar. These events caused unrest throughout Valacar's reign - it was believed that the pure Númenórean blood of the Royal House would be lost.
By the last years of Valacar's reign, certain southern provinces of Gondor saw this unrest break out into full rebellion. In the year Valacar died, III 1432, this rebellion became a civil war, a dark and bloody period known as the Kin-strife that would last for the next fifteen years, until Eldacar managed to secure his throne.
Valacar ruled as King of Gondor for sixty-six years, and was succeeded through great troubles by his son Eldacar.