Reigned 741 to 752. Year of birth unknown; died in March 752. Zachary sprang from a Greek family living in Calabria; his father, according to the , was called Polichronius. Most probably he was a deacon of the Roman Church and as such signed the decrees of the Roman council of 732. After the burial of his predecessor Gregory III on 29 November 741, he was immediately and unanimously elected pope and consecrated and enthroned on 5 December. His biographer in the describes him as a man of gentle and conciliatory character who was charitable towards the clergy and people. As a fact the new pope always showed himself to be shrewd and conciliatory in his actions and thus his undertakings were very successful. Soon after his elevation he notified Constantinople of his election; it is noticeable that his synodica (letter) was not addressed to the iconoclastic Patriarch Anastasius but to the Church of Constantinople. The envoys of the pope also brought a letter for the emperor. After the death of Leo III (18 June 741) his successor was his son Constantine V, Copronymus. However, in 742 Constantine’s brother-in-law Artabasdus raised a revolt against the new emperor and established himself in Constantinople; thus when the papal envoys reached Constantinople they found Artabasdus the ruler there. As late as 743 the papal letters were dated from the year of the reign of Constantine V; in 744, however, they are dated form the year of the reign of Artabasdus. Still the papal envoys do not seem to have come into close relations with the usurper at Constantinople, although the latter re-established the worship of images. After Constantine V had overthrown his rival, the envoys of the pope presented to him the papal letter in which Zachary exhorted the emperor to restore the doctrine and practice of the Church in respect to the worship of images. The emperor received the envoys in a friendly manner and presented the Roman Church with the villages of Nympha and Normia (Norba) in Italy, which with their territories extended to the sea.
When Zachary ascended the throne the position of the city and Duchy of Rome was a very serious one. Luitprand, King of the Lombards, was preparing a new incursion into Roman territory. Duke Trasamund of Spoleto, with whom Pope Gregory III had formed an alliance against Luitprand, did not keep his promise to aid the Romans in regaining the cities taken by the Lombards. Consequently Zachary abandoned the alliance with Trasamund and sought to protect the interests of Rome and Roman territory by personal influence over Luitprand. The pope went to Terni to see the Lombard king who received him with every mark of honour. Zachary was able to obtain from Luitprand that the four cities of Ameria, Horta, Polimartium, and Blera should be returned to the Romans, and that all the patrimonies of the Roman Church that the Lombards had taken from it within the last thirty years, should be given back; he was also able to conclude a truce for twenty years between the Roman Duchy and the Lombards. A chapel to the Saviour was built in the Church of St. Peter at Rome in the name of Luitprand, in which the deeds respecting this return of property were placed. After the pope’s return, the Roman people went in solemn procession to Saint Peter’s to thank God for the fortunate result of the pope’s efforts. Throughout the entire affair the pope appears as the secular ruler of Rome and the Roman territory. In the next year Luitprand made ready to attack the territory of Ravenna. The Byzantine exarch of Ravenna and the archbishop begged Pope Zachary to intervene. The latter first sent envoys to the Lombard king, and when these were unsuccessful he went himself to Ravenna and from there to Pavia to see Luitprand. The pope reached Pavia on the eve of the feast of Saints Peter and Paul. He celebrated the vigil and the feast of the princes of the Apostles at Pavia, and was able to induce the king to abandon the attack on Ravenna and to restore the territory belonging to the city itself. Luitprand died shortly after than and after his first successor Hildebrand was overthrown, Ratchis became King of the Lombards. The pope was on the best of terms with him. In 749 the new king confirmed the treaty of peace with the Roman Duchy. The same year Ratchis abdicated, with his wife and daughter took the monastic vows before the pope, and all three entered the monastic life.
In 743 Pope Zachary held a synod at Rome which was attended by sixty bishops. This synod issued fourteen canons on various matters of church discipline. On this occasion the pope took up the question of the impediments to marriage of relationship in the fourth degree, in regard to which the Germans claimed to have obtained a dispensation from Pope Gregory II. The year previous Zachary had written on this point to the bishops and kings of that province. An active correspondence was kept up between Zachary and Saint Boniface. The latter in his zealous labours had organized the Church in the German territories, and while doing this had kept in close connection with the Papal See. Early in 742, soon after his elevation, Zachary received a letter from Boniface in which the saint expressed his full submission to the possessor of the Chair of Peter and requested then confirmation of the three newly established Bishoprics of Wurzburg, Buraburg, and Erfurt; Boniface also sought authority to hold a synod in France and to suppress abuses in the lives of the clergy. The pope confirmed the three dioceses and commissioned Boniface to attend, as papal legate, the Frankish synod which Karlmann wished to hold. In a later letter Zachary confirmed the metropolitans of Rouen, Reims, and Sens appointed by Boniface, and also confirmed the condemnation of the two heretics Adelbert and Clement. Various questions in which the pope and Boniface disagreed were discussed in letters. In 745 was held the general synod for the Frankish kingdom called by Pepin and Carloman. Here decrees were passed against unworthy ecclesiastics, and the two heretics, Adelbert and Clement, were again condemned. Boniface sent a Frankish priest to Rome to make a report to the pope, and the latter held on 25 October 745, a synod at the Lateran at which, after exhaustive investigation, an anathema was pronounced against the two heretics. Zachary forwarded the acts of the synod with a letter to Boniface. Pepin and the Frankish bishops sent a list of questions respecting the discipline of the clergy and of the Christian population to Pope Zachary, and the latter answered in a letter of 746 in which decisions respecting the various points are given. These decisions were communicated to Boniface so that he might make them generally known at a Frankish synod. The following year, 747, Carloman resigned his authority and the world, went to Rome, and was received by Pope Zachary into a monastic order. At first he lived in the monastery on the Soracte, later at Monte Cassino. Thanks to the efforts of Saint Boniface all the Frankish bishops were now agreed in submission to the See of Saint Peter. Zachary sent still other letters to the bishops of Gaul and Germany, and also to Boniface as the papal legate for the Church of this region. Boniface was constantly in intercourse with Rome both by letters and envoys and sent important questions to the pope for decision. An important proof of the recognition by the Franks of the high moral power of the papacy is shown by the appeal to papal authority on the occasion of the overthrow of the Merovingian dynasty. Pepin’s ambassadors, Bishop Burkard of Wurzburg and Chaplain Folrad of Saint Denis, laid the question before Zachary: whether it seemed right to him that one should be king who did not really possess the royal power. The pope declared that this did not appear good to him, and on the authority of the pope Pepin considered himself justified in having himself proclaimed King of the Franks. The ecclesiastical activity of the pope also extended to England. Through his efforts the Synod of Cloveshove was held in 747 for the reform of church discipline in accordance with the advice given by the pope and in imitation of the Roman Church.
Zachary was very zealous in the restoration of the churches of Rome to which he made costly gifts. He also restored the Lateran palace and established several large domains as the settled landed possessions (domus cultæ) of the Roman Church. The pope translated to the Church of Saint George in Velabro the head of the martyr Saint George which was found during the repairs of the decayed Lateran Palace. He was very benevolent to the poor, to whom alms were given regularly from the papal palace. When merchants from Venice bought slaves at Rome in order to sell them again to the Saracens in Africa, the pope bought all the slaves, so that Christians should not become the property of heathens. Thus in a troubled era Zachary proved himself to be an excellent, capable, vigorous, and charitable successor of Peter. He also carried on theological studies and made a translation of the Dialogues of Gregory the Great into Greek, which was largely circulated in the East. After his death Zachary was buried in Saint Peters.