(16 Dec. 882–15 May 884)
Sometimes mistakenly listed as Martin II, he was son of a priest and born at Gallese, in Tuscany, entered the service of the Roman church when 12 years old, was made deacon by Nicholas I, and as such was sent to Constantinople by Nicholas in 866 as part of a delegation to negotiate over Bulgaria: they were turned back at the border. He later proved the most effective of three legates representing Hadrian II at the fourth council of Constantinople (869–70), which anathematized the recently deposed patriarch Photius (858–67; 878–86). On this occasion he had a sharp brush with Emperor Basil I (867–86) through refusing to depart from Hadrian's instructions. He became archdeacon and treasurer (arcarius) of the Roman church, and also bishop of Caere (now Cerveteri) in Etruria John VIII used him for difficult missions to the future Emperor Charles III (the Fat, 881–7) in Mar. 880, and in 882 to Athanasius of Naples, when he succeeded in breaking the bishop's alliance with the Saracens. When he succeeded John, he was the first bishop of another see to be elected pope in violation of the ancient canons (notably canon 15 of Nicaea) prohibiting the translations of bishops from one see to another (a prohibition to which Nicholas I had appealed when refusing to appoint Bishop Formosus of Porto, later to be pope, to the archbishopric of Bulgaria).
Marinus' election was carried through without consulting Charles III, but when the emperor visited Italy in June 883 Marinus met him at Nonantula, near Modena, secured his recognition, and held important discussions with him. One result of these was his decision to pardon Formosus of Porto and others accused of conspiring against John VIII, whom John had excommunicated and exiled, and to restore Formosus to his see and to release him from the vows he had sworn under duress to the pope. The belief that he refused to announce his election to Photius, reinstated as patriarch in 878, and that he and Photius excommunicated each other, is unfounded; Photius did his best to be reconciled to his former enemies, and Marinus went out of his way to retain Zacharias of Anagni, a friend of Photius and a supporter of pro-Greek policies at Rome, in the key position of papal librarian. A further result of his conversations with Charles III was that, exasperated by the pressure of Duke Guido III of Spoleto (crowned emperor in 891: d. 894) on the papal patrimony, he persuaded the emperor to pronounce a sentence of deposition on him.
Little else is known of Marinus' short reign, except that he had to settle a dispute between the archbishops of Reims and Sens over a newly founded monastery, and that he entertained excellent relations with Alfred the Great of England (849–99), out of regard for whom he exempted the Schola Saxonum, or English quarter in Rome, from taxes.
JW i. 425 f., ii. 704LP ii. 224PL 126: 966–70DBI lxx. 499–502 (I. Bonccorsi)DTC ix. 2476 f. (É. Amann)PRE xii. 340 (H. Böhmer)Levillain ii. 969–70 (F. Bougard)NCE ix. 167–8 (V. Gellhaus)Brezzi 83 f.Mann iii. 353–60Seppelt ii. 297 f., 331 f.J. Duhr, ‘Le Pape Martin I: Était-il évêque ou archidiacre lors de son élection?’, RSR24 (1934), 200–06