The Vatican's Annuario Pontificio (2003) identifies him as a Roman who reigned from 117 or 119 to 126 or 128. According to the Liberian Catalogue of popes, he ruled the Church during the reign of Hadrian "a consulatu Nigri et Aproniani usque Vero III et Ambibulo", that is, from 117 to 126.
Eusebius states in his Chronicon that Sixtus I was pope from 114 to 124, while his Historia Ecclesiastica, using a different catalogue of popes, claims his rule from 114 to 128. All authorities agree that he reigned about ten years.
The Felician Catalogue of popes and the various martyrologies style him martyr. St. Irenaeus of Lyons, writing in the second century, says nothing of his martyrdom; his successor Telesphorus is the first pope after St. Peter whom Irenaeus designates a martyr.
He was buried in the Vatican, beside the tomb of St. Peter. His relics are said to have been transferred to Alatri in 1132, though O. Jozzi ("Il corpo di S. Sisto I., papa e martire rivendicato alla basilica Vaticana", Rome, 1900) contends that they are still in the Vatican Basilica.
An image of Pope Saint Sixtus can be seen on a fresco at Chalivoy-Milon in the Berry and is reproduced here: http://picasaweb.google.com/JuliannaLees/ChalivoyMilon
Alban Butler (Lives of the Saints, 6 April) states that Clement X gave some of his relics to Cardinal de Retz, who placed them in the Abbey of St. Michael in Lorraine. The Xystus who is commemorated in the Catholic Canon of the Mass is Xystus II, not Xystus I.
His feast is celebrated on April 6.