Saint John I was born in Tuscany (the exact date has been lost) and served as a deacon in Rome. As deacon, he originally supported the AntiPope Laurentius but confessed his error in the support when he wrote a libelius to Pope Symmachus in 506. At the time he was only one of seven deacons in the church. He was elected Pope in 523.
As Pope, Saint John I ended the Acacian Schism (which had split the Patriarchate of Constantinople with the Roman See) and ratified the Alexandrian way of computing the date for Easter. Shortly after his election, the Arian King of the Ostrogoths in Italy sent him, despite many protests by the papacy, to the court of Justinian I to seek moderation of a decree that had recently been issued. King Theoderic threated him with no uncertain terms - should he fail to secure the moderation of Justin I's decree against Arians there would be severe repraisals against Christians who did not follow Arianism in the West. Saint John was succesfull in this mission and returned to Italy.
At Ravenna, King Theoderic's capital, Saint John I was arrested under the suspicion of conspiring with Justin I against King Theoderic. It was here in prison that Saint John I died of neglect and extremely poor conditions on May 18th, 526.
Today his feast day is celebrated on May 18th, the date of his death.