Here is the best I could find. It is from Wikipeda
The last traces of emperors being at the same time chief pontiffs are found in inscriptions of Valentinian, Valens, and Gratianus (Orelli, Inscript. n1117, 1118). From the time of Theodosius, the emperors no longer appear in the dignity of pontiff; but at last the title was assumed by the Christian bishop of Rome. The imperial title and the robes of pontifex maximus were given up by the Emperor Gratian(us) upon the influence of St. Ambrose. The reign of Gratian was also epochal since, in the year 382, state subsidies supporting the pagan religion were withdrawn and the Altar of Victory was removed from the Forum. Although the Emperor renounced the title, the office itself was never abolished. Pope Damasus I was the first Bishop of Rome to assume the title, referring to himself Pontifex Maximus in a petition to the Emperor for judicial immunity. From the reign of Theodosius, no power or office in late antiquity has claimed or contested the legitimacy of the Bishop of Rome from holding the the ancient Roman title and office of Pontifex Maximus.
It is also interesting to note the specific differentiation made by Emperor Theodosius on the law of 27 February 380 (De Fide Catolica) enacted in Thessalonica and published in Constantinople for the whole empire establishing Catholic Christianity as the official religion of the empire: …the profession of that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria… We authorize the followers of this law to assume the title Catholic Christians; but as for the others…. The implied differentiation is notable but the full title pontifex maximus is not explicitly in the text; pontificem is used instead, i.e.:…declarat quamque pontificem damasum sequi claret et petrum alexandriae episcopum*…*(Theodosian Code XVI.1.2; and Sozomen, “Ecclesiastical History”, VII, iv. ).
It is not clear if the title Pontifex Maximus was used consistently by the Bishops of Rome from the time it was relinquished by the Emperor Gratian (AD 376) onwards. This was a time when the declining Roman Empire was transitioning from pagan to Christian, and Constantinople would begin to assert itself to pre-eminence, historically leading to conflict with the Bishops of Rome. Near the final collapse of the Roman Empire with the invasion of the Huns and Vandals, Pope Leo the Great is also attributed to have assumed the title Pontifex Maximus. By the end of the 6th century, Gregory I was the first Pope to employ “Pontifex Maximus” in a formal sense, in a broader program of asserting Roman primacy. It has remained one of the titles of the popes to this day.
Despite its present character and transformation from its original Roman roots, this political or governing office has been in existence and in perpetual use since the birth of Rome. The pope is literally, for all intents and purposes and in reality, the ‘high-priest’ (which is another translation of pontifex maximus) of the Catholic religion. Therefore, in terms of its origins and not its usage, the title Pontifex Maximus is the oldest title being used by the pope.** It would also count as the oldest continuing Roman institution or European office spanning a history of almost 3,000 years**