List of 10 longest-reigning Popes

The list of 10 longest-reigning Popes:

Pius IX (1846-1878): 31 years and 7 months.
Leo XIII (1878-1903): 25 years and 5 months (9281 days).
John Paul II (1978-present): 25 years and 4 months or 9254 days as of February 16 2004, would pass Leo XIII on March 14 2004 if still reigning as Pope (and Pius IX in 2010)
Pope Pius VI (1775-1799): 24 years and 6 months (8962 days).
Adrian I (772-795): 23 years and 10 months.
Pius VII (1800-1823): 23 years and 5 months.
Alexander III (1159-1181): 21 years, 11 months, and 23 days.
St. Silvester I (314-335): 21 years and 11 months.
St. Leo I (440-461): 21 years and 1 month.
Urban VIII (1623-1644): almost 20 years

St. Peter (30 to 64/67 AD): 34 or 37 years, of which, according to tradition, he spent 25 in Rome. (Peter was not however known as pope in his lifetime)

See also List of 10 shortest-reigning Popes