On June 15th, 1871, Pius IX (r. 1846-71) became the first pope to reign for twenty-five years. The achievement was duly commemorated in St Peter's Basilica when his portrait (in mosaic) was placed above a much venerated bronze statue of St Peter.
The inscription reads: "The Vatican clergy embellished the sacred seat for Pius IX, Pontifex Maximus, who alone in the Roman pontificate equalled the years of St Peter on the sixteenth day before the Kalends of July, 1871."
According to a widely-held belief, St Peter reigned as the first pope for exactly twenty-five years.
In addition to Pius IX, only two other popes have reigned for twenty-five years or more: Leo XIII (r. 1878-1903) and John Paul II (r. 1978-2005).
Pius IX also holds the record for being the longest-reigning pope in the history of the papacy. His reign lasted a grand total of 31 years and 236 days.