John III (Latin Iohannes; original name Catelinus) was elected Bishop of Rome on July 17, 561, four months after the death of Pelagius I. His thirteen-year pontificate was principally devoted to two areas. First, the gradual ongoing pastoral work of reconciling the Western Catholic Three Chapters Schism: the substantial bloc of Western Catholic bishops (particularly in northern Italy, Gaul, Spain, and Britain) who had refused to enter into communion with Pelagius I because of his ratification of the Justinianic Three Chapters condemnation. John III's careful pastoral diplomacy across the 560s and early 570s gradually reconciled most of the Western Catholic episcopate, though the northern Italian (Aquileian) Three Chapters Schism continued in low intensity until the early 8th century.
Second, the substantial architectural work at the Roman catacombs: John III is recorded by the Liber Pontificalis as the foundational sponsor of the post-Gothic-War restoration of the major Roman catacomb-sanctuary system (which had been substantially damaged during the long Gothic-Byzantine wars of 535–554). He restored the foundational tomb-shrines of the Roman martyrs at the principal Roman catacombs (San Callisto, Sant'Agnese, Santa Priscilla, San Pancrazio).
He died at Rome on July 13, 574 — during the period of substantial Lombard pressure on the city after the Lombard invasion of Italy of 568.