A relative of two previous popes, Stephen V and Sergius II, he had been called to the papacy twice before but declined. He accepted the call on Dec. 14, 867. Under his vigorous predecessor, St. Nicholas I, the papacy had reached a high point that Adrian could not maintain. Vacillatory and lacking continuity, he was snubbed by Charles II the Bald, king of France. He readmitted King Lothar II (Lothar (II)) of Lorraine to communion, but Lothar's early death (869) created a difficult problem of succession in which Adrian ineffectually intervened. Adrian also had difficulties with the powerful Archbishop Hincmar of Reims, Fr., by steadfastly upholding the unlimited right of bishops to appeal to the pope.

Adrian approved the use of the Slavic language in liturgy by Saints Cyril and Methodius. By making Methodius archbishop of Sirmium, Adrian won the Moravians' faithfulness.

Adrian's legates took part in the eighth ecumenical council and the fourth Council of Constantinople (Constantinople, Council of) (869–870), which deposed the Byzantine patriarch Photius. For the sake of reuniting East with West, Adrian accepted the council's 21st canon, which gave the patriarch of Constantinople rank second to that of the Roman see. He refused, however, to sanction the transfer of the Bulgarians to the patriarchate of Constantinople, and Bulgaria was lost to the Roman Catholic church during Adrian's pontificate.