Early life
To a spiritually exhausted and politically uncertain Western Europe beset by Muslim and Norse incursions, Pope Nicholas appeared as a conscientious representative of the Roman primacy in the Church. He was filled with a high conception of his mission for the vindication of Christian morality, the defence of God's law against powerful, worldly bishops.

Papacy
Archbishop John of Ravenna oppressed the inhabitants of the papal territory, treated his suffragan bishops with violence, made unjust demands upon them for money, and illegally imprisoned priests. He also forged documents to support his claims against the Roman See and maltreated the papal legates. As the warnings of the pope were without result, and the archbishop ignored a thrice-repeated summons to appear before the papal tribunal, he was excommunicated. Having first visited the Emperor Louis at Pavia, the archbishop repaired, with two imperial delegates to Rome, where Nicholas cited him before the Roman synod assembled in the autumn of 860. Upon this John fled from Rome.
Going in person to Ravenna, the pope then investigated and equitably regulated everything. Again appealing to the emperor, the archbishop was recommended by him to submit to the pope, which he did at the Roman Synod of November, 861. Later on, however, he entered into a pact with the excommunicated archbishops of Trier and Cologne, was himself again excommunicated, and once more forced to make his submission to the pope. Another conflict arose between Nicholas and Archbishop Hincmar of Reims: this concerned the prerogatives of the papacy. Bishop Rothad of Soissons had appealed to the pope against the decision of the Synod of Soissons, of 861, which had deposed him; Hincmar opposed the appeal to the pope, but eventually had to acknowledge the right of the papacy to take cognizance of important legal causes (causae majores) and pass independent judgment upon them. A further dispute broke out between Hincmar and the pope as to the elevation of the cleric Wulfad to the archiepiscopal See of Bourges, but here, again, Hincmar finally submitted to the decrees of the Apostolic See, and the Frankish synods passed corresponding ordinances.

Bishops
Nicholas showed the same zeal in other efforts to maintain ecclesiastical discipline, especially as to the marriage laws. Ingiltrud, wife of Count Boso, had left her husband for a paramour; Nicholas commanded the bishops in the dominions of Charles the Bold to excommunicate her unless she returned to her husband. As she paid no attention to the summons to appear before the Synod of Milan in 860, she was put under the ban.
The pope was also involved in a desperate struggle with Lothair II of Lorraine over the inviolability of marriage. Lothair had abandoned his lawful wife Theutberga to marry Waldrada. At the Synod of Aachen, 28 April 862, the bishops of Lorraine approved this union, contrary to ecclesiastical law. At the Synod of Metz, June, 863, the papal legates, bribed by the king, assented to the Aachen decision, and condemned the absent Theutberga. Upon this the pope brought the matter before his own tribunal. The two archbishops, Günther of Cologne and Thietgaud of Trier, who had come to Rome as delegates, were summoned before the Lateran Synod of October, 863, when the pope condemned and deposed them as well as John of Ravenna and Hagano of Bergamo. The Emperor Louis II took up the cause of the deposed bishops, while King Lothair advanced upon Rome with an army and laid siege to the city, so that the pope was confined for two days in St. Peter's without food. Yet Nicholas did not waver in his determination; after being reconciled with the pope, the emperor withdrew from Rome and commanded the former Archbishops of Trier and Cologne to return to their homes. Nicholas never ceased from his efforts to bring about a reconciliation between Lothair and his lawful wife, but without effect.
Another matrimonial case in which Nicholas interposed was that of Judith, daughter of Charles the Bold, who had married Baldwin, Count of Flanders, without her father's consent. Frankish bishops had excommunicated Judith, and Hincmar of Reims had taken sides against her, but Nicholas urged leniency, in order to protect freedom of marriage.

Relations with the Eastern Church
He encouraged the missionary activity of the Church. He sanctioned the union of the Sees of Bremen and Hamburg, and confirmed to St. Anschar, Archbishop of Bremen, and his successors the office of papal legate to the Danes, Swedes, and Slavs. In many other ecclesiastical matters, he issued letters and decisions, and he took active measures against bishops who were neglectful of their duties.
At Rome, Nicholas rebuilt and endowed several churches, and constantly sought to encourage religious life. His led a pious personal life guided by a spirit of Christian asceticism. Regino of Prüm reports that Nicholas was highly esteemed by the citizens of Rome and by his contemporaries generally (Chronicon, "ad annum 868," in "Mon. Germ. Hist." Script.", I.579), and after death was regarded as a saint.
A much discussed question and one that is important in judging the position taken by this pope is whether he made use of the forged pseudo-Isidorian papal decretals. After exhaustive investigation, Schrörs has decided that the pope was neither acquainted with the pseudo-Isidorian collection in its entire extent, nor did he make use of its individual parts; that he had perhaps a general knowledge of the false decretals, but did not base his view of the law upon them, and that he owed his knowledge of them solely to documents which came to him from the Frankish Empire [Schrörs, "Papst Nikolaus I. und Pseudo-Isidor" in Historisches Jahrbuch, XXV (1904), 1 sqq.; Idem, "Die pseudoisidorische 'Exceptio spolii' bei Papst Nikolaus I" in Historisches Jahrbuch, XXVI (1905), 275 sqq.].