Ioannes 21 was bishop of Portus; he was one of the hundred and twenty-five Western bishops who attended the Council of Rome at Easter 680, and who subscribed the decree condemning monotheletism: Riedinger II 2. 144, lines 14-15 (= Mansi XI 301-302) (Ἰωάννης ἐλάχιστος ἐπίσκοπος τῆς ἁγίας ἐκκλησίας τοῦ Πόρτου; in the old Latin version printed in Riedinger II 2. 145, line 13, his see is called "ecclesiae Portuensis"). Later in 680, he attended the Third Council of Constantinople (the Sixth Ecumenical Council) as one of three Italian bishops representing the hundred and twenty-five bishops of the Council of Rome (the two others were Aboundantios 1 and Ioannes 32); he attended all eighteen sessions of the Council: Riedinger II 2. 16-822 (= Mansi XI 209-669). He was sent to the Council by pope Agatho 1: Riedinger II 2. 56, line 7, 895, line 30 (= Mansi XI 236, 716). Bishop of Portus; sent by Agatho 1 as one of the papal representatives to the Council: Lib. Pont. 81. 3, cf. 82. 2 ("legati sedis apostolicae"), Paul. Diac., Hist. Lang. VI 4 (Agatho sent two "legati, Iohannes diaconus et Iohannes Portuensis episcopus"). In the subscriptions to the statement of the faith, which condemned monotheletism, and to the Council, he is styled Ἰωάννης ἀνάξιος ἐπίσκοπος τῆς ἁγίας ἐκκλησίας Πόρτου καὶ ληγάτος πάσης τῆς συνόδου τῆς ἁγίας καὶ ἀποστολικῆς ἐκκλησίας τοῦ ἀποστολικοῦ θρόνου πόλεως Ῥώμης (or similar): Riedinger II 2. 778, lines 23-25, 822, lines 28-29, cf. II 2. 891, lines 16-18 (= Mansi XI 641, 669, cf. XI 688). In the lists of those attending each session he is styled Ἰωάννου ἐπισκόπου πόλεως Πόρτου or similar: Riedinger II 2. 16, line 13, 28, line 17, etc. (= Mansi XI 209, 217, etc.). At the twelfth session (22 March) he compared a letter in Latin from pope Honorios from the patriarchal archives at Constantinople with a copy brought before the Council and found them to correspond: Riedinger II 2. 564 (= Mansi XI 548). He was among those who signed the copy of the definition of the faith sent to the bishop of Rome, Agatho 1: Riedinger II 2. 891, lines 16-18 (= Mansi XI 688-689). During the Council he celebrated mass in Latin in Hagia Sophia in the presence of the emperor and the patriarchs (see Georgios 1); the date was the Sunday of the Octave of Easter: Lib. Pont. 81. 15. In 681 this fell on Sunday, 21 April, and therefore fell between the fourteenth and fifteenth sessions of the Council; cf. Duchesne, Lib. Pont., p. 358, n. 33. In 682 he was one of the three bishops who, because the see of Albanum was vacant, consecrated Leo II (Leo 16) as bishop of Rome: Lib. Pont. 82. 7 (the others were Andreas 6 of Ostia and Plakentios 1 of Velitrae). The same three perhaps consecrated John V (Ioannes 31) as bishop of Rome in 685: Lib. Pont. 84. 3 (their names are not given, only their sees). After the Quinisext Council (in 692) he and the papal consiliarius Bonifatios 2 were compelled to go to Constantinople by the emperor's agent, Sergios 45, when the pope Sergius (Sergios 30) refused to accept the decisions of the Council: Lib. Pont. 86. 7 ("Iohannem Deo amabilem Portuensem episcopum").

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