Leontios 9 was a basilikos asekretis (Gloriosissimus imperialis asecretis); he is attested at the first, second, fourth, fifth and sixth sessions of the Second Council of Nikaia (the Seventh Ecumenical Council) in 787; he is styled ὁ ἐνδοξότατος (or καθωσιώμενος) ἀσηκρῆτις Λεόντιος or similar: Mansi XII 1002, 1023, 1054 (Λεόντιος ὁ εὐκλεέστατος βασιλικὸς ἀσηκρῆτις), XIII 4, 157 (Λεόντιος ὁ καθωσιώμενος βασιλικὸς ἀσηκρῆτις), 184 (Λεόντιος ὁ καθωσιωμένος ἀσηκρῆτης), 204 (Λέων ὁ εὐκλεέστατος ἀσηκρῆτις; the Latin version calls him "Leontius inclytus a secretis"). At the first session, on 24 September 787, he read out a letter from the emperors Constantine VI (Konstantinos 8) and Eirene 1 to the Council: Mansi XII 1002. Later in this session, while a letter of St Athanasios was being produced, he intervened during the interval to ask that the reading of letters of St Basil be continued: Mansi XII 1023. At the second session, on 26 September 787, he asked for the letters from the bishop of Rome, Hadrian I (Hadrianos 1), and from the oriental patriarchs, mentioned in the letter from the emperors, to be produced: Mansi XII 1054. At the fourth session he announced that the books of the Fathers requested by the Council were available and he himself began the readings with the Bible: Mansi XIII 4. At the fifth session, on 4 October 787, he announced that the works by the Fathers and by heretics requested by the Council were available to be read out: Mansi XIII 157-160. During this session a copy of a homily on martyrs by the deacon Konstantinos 76 was produced and Leontios 9 drew attention to the curious fact that the iconoclasts had cut out a number of pages containing references to icons but had left untouched the covers which were of silver and displayed images of saints: Mansi XIII 184. At the start of the sixth session, on 11 October 787, he reminded the Council that the previous session had heard writings by the heretics and informed them that at the present one they had a copy of the statement of faith adopted by the iconoclast Council (of 754) and a refutation of it, ready for reading if the Council wished: Mansi XIII 204.

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