From Catholic Encyclopedia 1913:
Died at Ravenna on 18 or 19 May (according to the most popular calculation), 526. A Tuscan by birth and the son of Constantius, he was, after an interregnum of seven days, elected on 13 August, 523, and occupied the Apostolic see for two years, nine months, and seven days.
We know nothing of the matter of his administration, for his Bullarium contains only the two letters addressed to an Archbishop Zacharias and to the bishops of Italy respectively, and it is very certain that both are apocryphal.
We possess information -- though unfortunately very vague -- only about his journey to Constantinople, a journey which appears to have had results of great importance, and which was the cause of his death. The Emperor Justin, in his zeal for orthodoxy, had issued in 523 a severe decree against the Arians, compelling them, among other things, to surrender to the Catholics the churches which they occupied. Theodoric, King of the Ostrogoths and of Italy, the ardent defender of Arianism, keenly resented these measures directed against his coreligionists in the Orient, and was moreover highly displeased at seeing the progress of a mutual understanding between the Latin and Greek Churches, such as might favour certain secret dealings between the Roman senators and the Byzantine Court, aiming at the re-establishment of the imperial authority in Italy. To bring pressure to bear upon the emperor, and force him to moderate his policy of repression in regard to the heretics, Theodoric sent to him early in 525 an embassy composed of Roman senators, of which he obliged the pope to assume the direction, and imposed on the latter the task of securing a withdrawal of the Edict of 523 and -- if we are to believe "Anonymous Valesianus" -- of even urging the emperor to facilitate the return to Arianism of the Arians who had been converted.
There has been much discussion as to the part played by John I in this affair. The sources which enable us to study the subject are far from explicit and may be reduced to four in number: "Anonymous Valesianus", already cited; the "Liber Pontificalis"; Gregory of Tours's "Liber in gloria martyrum"; and the "Liber Pontificalis Ecclesiæ Ravennatis". But it is beyond question that the pope could only counsel Justin to use gentleness and discretion towards the Arians; his position as head of the Church prevented his inviting the emperor to favour heresy. That this analysis of the situation is correct is evident from the reception which the pope was accorded in the East -- a reception which certainly would not have been kindly, had the Roman ambassadors opposed the emperor and this Catholic subjects in their struggle waged against the Arian sect. The inhabitants of Constantinople went out in throngs to meet John. The Emperor Justin on meeting him prostrated himself, and, some time afterwards, he had himself crowned by the pope. All the patriarchs of the East made haste to manifest their communion in the Faith with the supreme pontiff; only Timothy of Alexandria, who had shown himself hostile to the Council of Chalcedon, held aloof. Finally, the pope, exercising his right of precedence over Epiphanius, Patriarch of Constantinople, solemnly officiated at St. Sophia in the Latin Rite on Easter Day, 19 April, 526. Immediately afterwards he made his way back to the West.
If this brilliant reception of John I by the emperor, the clergy, and the faithful of the Orient proves that he had not been wanting in his task as supreme pastor of the Church, the strongly contrasting behaviour of Theodoric towards him on his return is no less evident proof. This monarch, enraged at seeing the national party reviving in Italy, had just stained his hands with the murder of Boethius, the great philosopher, and of Symmachus his father-in-law. He was exasperated against the pope, whose embassy had obtained a success very different from that which he, Theodoric, desired and whom, moreover, he suspected of favouring the defenders of the ancient liberty of Rome. As soon as John, returning from the East, had landed in Italy, Theodoric caused him to be arrested and incarcerated at Ravenna. Worn out by the fatigues of the journey, and subjected to severe privations, John soon died in prison.
His body was transported to Rome and buried in the Basilica of St. Peter. In his epitaph there is no allusion to his historical role. The Latin Church has placed him among its martyrs, and commemorates him on 27 May, the ninth lesson in the Roman Breviary for that date being consecrated to him.
St. John, Pope I18-May
From Catholic Encyclopedia 1913: