Leo 21 was bishop of Mesembria (in Haemimontus); in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 994-XIII 486. In the lists of those present at the first and seventh sessions he is styled Λέοντος ἐπισκόπου Μεσημβρίας: Mansi XII 994, XIII 368. Elsewhere he is styled Λέων ὁ ὁσιώτατος ἐπίσκοπος Μεσημβρίας or similar: Mansi XII 1098, XIII 141, XIII 385. At the second session he spoke in support of the traditional veneration of icons as described by pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1098. At the fourth session he subscribed to the statements read out from the Fathers in support of icon veneration: Mansi XIII 141. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 385.

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