Basil came of a peasant family that had settled in Macedonia, perhaps of Armenian origin. He was a handsome and physically powerful man who gained employment in influential official circles in Constantinople and was fortunate enough to attract the imperial eye of the reigning emperor, Michael III. After rapid promotion he became chief equerry, then chamberlain, and finally, in 866, co-emperor with Michael. Quick to sense opposition, he forestalled the Emperor's uncle, the powerful Caesar Bardas, by murdering him (866) and followed this by killing his own patron, Michael, who had begun to show signs of withdrawing his favour (867).

From the mid-9th century onward, the Byzantines had taken the offensive in the agelong struggle between Christian and Muslim on the eastern borders of Asia Minor. Basil continued the attacks made during Michael III's reign against the Arabs and their allies, the Paulicians (Paulician), and had some success. Raids across the eastern frontier into the Euphrates region continued, though Basil did not manage to take the key city of Melitene. But the dangerous heretical Paulicians on the borders of the Armenian province in Asia Minor were crushed by 872, largely owing to the efforts of Basil's son-in-law Christopher. In Cilicia, in southeast Asia Minor, the advance against the Emir of Tarsus succeeded under the gifted general Nicephorus Phocas the Elder. Though Constantinople had lost much of its former naval supremacy in the Mediterranean, it still had an effective fleet. Cyprus appears to have been regained for several years.

Basil's plans for Italy involved him in negotiations with the Frankish emperor Louis II, the great-grandson of Charlemagne. The Byzantine position in southern Italy was strengthened with the help of the Lombard duchy of Benevento, and the campaigns of Nicephorus Phocas the Elder did much to consolidate this. The region was organized into the provinces of Calabria and Langobardia. But key cities in Sicily, such as Syracuse in 878, still continued to fall into Muslim hands, an indication of the strength of Arab forces in the Mediterranean.

Another arm of Byzantine policy was the attempt to establish some measure of control over the Slavs in the Balkans. Closely allied to this was the delicate question of ecclesiastical relations between Constantinople and Rome. During Basil I's reign, the young Bulgar state accepted the ecclesiastical jurisdiction of Constantinople (870). This had significant results both for the Balkan principalities and for the Orthodox Church, as well as greatly strengthening Byzantine influence in the south Slav world. Basil had inherited a quarrel between Photius (Photius, Saint) and Ignatius as to which was to be patriarch of Constantinople. This had international implications, since appeals had been made to Rome. Immediately on his accession, Basil attempted to win support at home and to conciliate Rome by reinstating the deposed patriarch Ignatius and excommunicating Photius. Eventually, Photius was restored by Basil on the death of Ignatius (877) and recognized by Rome in 879. Contrary to the belief that used to be held, no “second schism” occurred. Basil successfully resolved the tension between liberal and strict Byzantine churchmen and managed to maintain a show of peace between East and West despite Rome's displeasure at the marked extension of imperial influence in the new Balkan principalities.

Toward the end of his life, Basil seemed to suffer fits of derangement, and he was cruelly biassed against his son Leo. Basil died on the hunting field. The 11th-century historian Psellus wrote of his dynasty as “more blessed by God than any other family known to me, though rooted in murder and bloodshed.” But Macedonian historians were understandably biassed in favour of the existing dynasty, to the detriment of the rulers it had supplanted. Recent historical research has raised the stature of Basil's predecessor, Michael III, and his regents. It is now generally agreed that the “new age” in Byzantine history began with Michael III in 842 and not with the Macedonian dynasty in 867. Basil's policies were largely determined, both at home and abroad, by factors not of his own making.