Became king of Judah at the age of 8 in 639 BCE at a time when the power of Assyria was contracting, which enabled him both to expand the territory and to institute important religious reforms, said to be prompted by the discovery of a law book in the Temple (2 Kgs. 23) usually thought to be part of the book of Deuteronomy in some form. Religious practice was centralized in Jerusalem and this was inaugurated with a great Passover celebration.

Josiah was killed at the battle of Megiddo in 609 BCE when he intervened to prevent the Egyptians from assisting the weakened Assyrians at a time when they were being threatened by the rising power of Babylonia (2 Kgs. 23: 29).