Marinus I — sometimes confused in older numbering with the Pope Martin II — was the first pope in Catholic history to have already been the bishop of another see before his papal election: he was Bishop of Caere (Cerveteri) when elected to the papal throne on December 16, 882, immediately after the assassination of John VIII.
His translation from one diocesan see to another — though controversial in canon law (which generally forbade episcopal translation in the early-medieval period) — established the precedent that would become the medieval and modern Catholic norm.
His two-year pontificate (882–884) was devoted to the stabilization of the Roman situation after the assassination of John VIII. He confirmed the Anglo-Saxon King Alfred the Great's Schola Anglorum (the Anglo-Saxon hostel in Rome for English pilgrims) and exempted it from the Roman tribute, a diplomatic favour to the Anglo-Saxon-Wessex Catholic kingdom in its hour of Viking crisis. He rehabilitated the previously-condemned Formosus of Porto (who would become Pope Formosus in 891, and whose corpse would be the object of the notorious Cadaver Synod of 897).
Marinus I died on May 15, 884. His pontificate was as the transition between the dramatic John VIII era and the saeculum obscurum.