He was the third son of John Komnenos, the doux (governor) of the theme of Dyrrhachium, and his wife Anna Dalassene and spent his youth and early manhood as a soldier on the eastern frontier. He came to power following a rebellion against his uncle, the usurper Nikephoros III Botaneiates, whom he captured and replaced in 1081.
His reign began with a string of military victories. He repelled the Normans of Robert Guiscard in 1081 and conquered Dyrrhachium the next year. In 1091 he defeated the Pechenegs in a famous battle at Levounion. He also led two military campaigns against the Seljuk Turks, both of which ended with Byzantine defeats (at the Battle of Mantzikert in 1071, he was a captive of the Seljuks). The Byzantines regained Nicaea in 1087 and most of the Aegean coast in 1091.
The most lasting result of Alexios' reign was the establishment of the Komnenian dynasty. Under the Komnenoi, the Byzantine Empire experienced a period of political stability and military success that culminated in the restoration of large territories in the 12th century.
He was also a major benefactor of the Orthodox Christian Church. He promoted monasticism and theological education and founded several churches and monasteries, including the Panagia Blachernarum and the Monastery of Christ Pantokrator. Alexios also played an important role in promoting the Crusades, which would eventually lead to the capture of Jerusalem by the European Christians in 1099.
Alexios died in Constantinople in 1118 and was succeeded by his son, John II Komnenos. He is regarded as one of the greatest emperors of the Byzantine Empire and is remembered for his military successes, his promotion of the arts and sciences, and his strong support of the Orthodox Church.