Albert Barnes Commentary 2 Kings 12:2

"And Jehoash did that which was right in the eyes of Jehovah all his days wherein Jehoiada the priest instructed him." — 2 Kings 12:2 (ASV)

All his days ... - That is, as long as Jehoiada was his adviser (compare 2 Chronicles 24:15–22). Jehoiada was, practically speaking, regent during the minority of Jehoash, a period of 10 or 12 years.

An increase of power for the priestly order was the natural consequence. Jehoiada bore the title of “high priest” (2 Kings 12:10), a title that had been out of use since the time of Eleazar (Joshua 20:6). From this time on, the Levitical order became more involved in public affairs and possessed greater influence than previously. Jehoiada’s successors traced their office to him rather than to Aaron (Jeremiah 29:26).