Marinos 15 was a deacon at Rome ("Marinum sanctae sedis diaconum apostolicae; apostolicae sedis minister"); in 866 he accompanied the bishop of Ostia, Donatus (Donatos 2), and the priest Leo 164 on a mission from the bishop of Rome, Nikolaos 28, to deliver the pope's judgment on the affair of the patriarchs Photius (Photios 1) and Ignatios 1 to Constantinople; they travelled through Bulgaria (cf. also Formosus 1 and Paulos 57), but were held up at the frontier of Byzantium by a local official, Theodoros 181, and prevented from going further; they waited for forty days, suffering insults and abuse, and then learned that the emperor had forbidden them entry; they therefore returned to Rome to report what had happened: Lib. Pont. 107. 70-72.

One of the seven deacons at Rome; in 869 he and the two bishops Stephanos 89 and Donatos 2 represented the bishop of Rome, Hadrian II (Hadrianos 8), at the Council of Constantinople summoned by Basil I (Basilios 7) and Ignatios 1: Nicetas, Vita Ignatii 544C (Μαρῖνος εἷς τῶν ἑπτὰ διακόνων). He is to be identified with Marinos, the bishop of Castella, to whom the patriarch Photius (Photios 1) addressed a letter in March/April 880: Photius, Ep. 272 (II 222 Laourdas-Westerink) (addressed Μαρίνῳ ἐπισκόπῳ Καστέλλης Ῥωμαίων). The letter indicated a reconciliation between the two.

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