Outside the Gospels, Caiaphas is known chiefly from Josephus, whose Antiquities of the Jews gives the high-priestly succession of the period and anchors him in datable history. Josephus records that "Joseph, who was called Caiaphas," was appointed to the high priesthood by the Roman prefect Valerius Gratus, and later removed by the legate Vitellius, about 36 CE. Combined with the tenure of Gratus, this places Caiaphas's pontificate from roughly 18 to 36 CE.
That span is remarkable. High priests in this period were appointed and dismissed by Rome, and most held office only a few years; Caiaphas's eighteen-year tenure, overlapping almost exactly with the prefecture of Pontius Pilate, implies a skilful operator who kept the confidence of successive Roman governors through a turbulent time. The Gospel picture of a high priest working in close concert with the Roman authority to remove a perceived threat is, in that light, historically plausible.
Josephus thus supplies what the Gospels cannot: an independent, dated framework for the man. He confirms that a high priest named Joseph Caiaphas held office in Jerusalem in precisely the years the Gospels require, giving the Passion narrative one of its firmest points of contact with the wider record of the period.