Dior's grandfather Hador was the last of the Stewards to live a truly long life, and he ruled Gondor for one hundred and seventeen years. This meant that his son Barahir, and his grandson Dior, both came late to the Stewardship and ruled for only a brief time. Dior was the last Steward to enjoy an untroubled rule, as his heir Denethor would see the breaking of the Watchful Peace and the beginning of centuries of warfare.
Dior is the first of only two recorded Ruling Stewards to leave no direct heir. In volume XII of The History of Middle-earth we're told that he had no children of his own, and was succeeded therefore by the son of his sister Rían.
Dior ruled as Steward for twenty-three years, and was succeeded by his nephew Denethor I.
The date of Dior's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.