Caiaphas was appointed High Priest by Valerius Gratus, the Procurator of Judah prior to Pontius Pilate in 18AD. During Valerius Gratus tenure as Procurator he auctioned off the High Priesthood to the highest bidder on five occasions. As a result it was only the wealthy, corrupt and pro-Roman who could become High Priest during this time.
In the fifteenth year of the reign of Tiberius Caesar, when Pontius Pilate was governor over Judah, Herod Antipas, another son of King Herod, was ruler over Galilee, his half-brother Philip was ruler over Iturea and Traconitis, Lysanias was ruler over Abilene and Annas and Caiaphas were the High Priests, the word of God came to John in the wilderness and he began his public ministry.
The Torah states that the High Priesthood is a lifetime position and since there were five High Priests still living at this time they all retained the title of ‘High Priest’. This could make things confusing as we see in the Gospel of Luke where it says there were two High Priests in office at the time John started his ministry – Annas and Caiaphas. There may have been five living High Priests there could however be only one person who served as the reigning High Priest at a time and during the period of the Gospel story (28-34AD) it was Caiaphas.
The reason was that Annas, the father-in-law to Caiaphas, was the one who bankrolled the purchase the High Priesthood for his son-in-law. There was obviously good money to be made in the ‘business’ of the priesthood. So not only did Annas keep the title of High Priest but also he obviously still retained considerable influence over the political and economic spheres of the Sanhedrin. Hence the reason Luke refers to Annas as a co-reigning High Priest.
Caiaphas held the position from 18AD until his death in 37AD.