He was the son of the Patrician and Prefect of Constantinople, George, and his wife Eukratia, and entered the service of the State. In 784 when Paul IV Patriarch of Constantinople died Tarasius was an imperial secretary, and a champion of the veneration of images. It may be that before his death the patriarch had recommended Tarasius as his successor in the patriarchate to the Empress Irene who was regent for her son Constantine VI (780-797). After the burial of Paul IV a great popular assembly was held before the Magnaura Palace to discuss the filling of the vacant see. The empress delivered an oration on the new appointment to the patriarchate and the people proclaimed Tarasius as the most worthy candidate. The empress agreed but said that Tarasius refused to accept the position. Tarasius now made a speech himself in which he declared he felt himself unworthy of the office, further that the elevation of a layman was very hazardous, and that the position of the Church of Constantinople had become a very difficult one, as it was separated from the Catholics of Western Europe and isolated from the other Oriental patriarchates; consequently he would only be willing to accept the position of patriarch on condition that Church unity be restored and that, in connection with the pope, an oecumenical council be called. The majority of the populace approved of these views and the imperial Court agreed to it. So on 25 December, 784, Tarasius was consecrated patriarch. In 785 he sent the priest George as his legate to Hadrian I with a letter in which he announced his appointment. In his reply the pope expressed his disapproval of the elevation of Tarasius directly from the laity to the dignity of a bishop contrary to canonical regulation, but allowed clemency to rule in view of the orthodoxy of the new patriarch's views, and recognized him as patriarch. After this by joint action with the pope and the imperial Court Tarasius called the Second Council of Nicaea, the Seventh Ecumenical Council, which rejected Iconoclasm. Union with the Roman Church was restored.
After the synod the patriarch had a number of struggles not only with the Iconoclastic party of the capital but also with a party of Orthodox monks. First, the latter upbraided him for restoring to office the bishops who had formerly maintained Iconoclasm, but who had submitted to the decrees of the Council of 787. As, however, this was in accordance with the decrees of the council the accusation was allowed to drop. Another accusation was much more serious, namely, that Tarasius tolerated and encouraged simony, because those bishops who had given money to obtain their positions were only commanded by him to do a year's penance and were permitted to retain their offices. The patriarch defended himself in writing against this accusation which he denied in toto; moreover, he issued a severe synodal letter against Simonists. The monks, however, were not satisfied; they maintained their accusations and also attacked the Council of 787. At a later date Theodore of Studium, who took part in these disputes, changed his opinion of Tarasius, and also of the Second Council of Nicaea, the oecumenical character of which he acknowledged. Many serious difficulties still existed in regard to Western Europe. There were also fresh disputes in Constantinople when the Emperor Constantine VI put aside his lawful wife and wished to marry Theodata, a relative of Abbot Theodore of Studium. Tarasius positively refused to perform the second marriage and expressed his displeasure at the conduct of the priest Joseph who had married the emperor. The zealous monks, whose leaders were the Abbots Plato of Saccudium and Theodore of Studium, accused the patriarch of weakness, because he took no further steps against the emperor. They refused to have Church fellowship any longer with Tarasius, and were, consequently, violently persecuted by the emperor who, however, also treated the patriarch harshly. After Irene had dethroned Constantine in 797, Tarasius deposed the priest Joseph and peace was once more restored between the patriarch and the monks. (See THEODORE OF STUDIUM). In 802 Tarasius crowned as emperor Nicephorus, who had overthrown Irene, an act that greatly dissatisfied the populace. The patriarch had nothing to do with the intrigues of the court. His life was ascetic and simple, he checked the luxury of the clergy, preached with great zeal, and was very benevolent to the poor. After his death he was venerated as a saint. His name is also placed in the Roman Martyrology under the date of 25 February. Catholic Enclopedia