Andronicus I Comnenus (c.1118-1185), Byzantine emperor, son of prince Isaac Comnenus, and grandson of Alexius I Comnenus, was born about the beginning of the 12th century. His birth has been estimated to c. 1118. He was endowed by nature with the most remarkable gifts both of mind and body. He was handsome and eloquent, but licentious; and at the same time active, hardy, courageous, a great general and an able politician.

Andronicus' early years were spent in alternate pleasure and military service. In 1141 he was taken captive by the Seljuk Turks and remained in their hands for a year. On being ransomed he went to Constantinople, where was held the court of his cousin, the emperor Manuel, with whom he was a great favourite. Here the charms of his niece, the princess Eudoxia, attracted him. She became his mistress, while her sister Theodora stood in a similar relation to the emperor Manuel.

In 1152, accompanied by Eudoxia, he set out for an important command in Cilicia. Failing in his principal enterprise, an attack upon Mopsuestia, he returned, but was again appointed to the command of a province. This second post he seems also to have left after a short interval, for he appeared again in Constantinople, and narrowly escaped death at the hands of the brothers of Eudoxia.

About this time (1153) a conspiracy against the emperor, in which Andronicus participated, was discovered and he was thrown into prison. There he remained for about twelve years, during which time he made repeated but unsuccessful attempts to escape. At last, in 1165, he was successful; and, after passing through many dangers, reached the court of Yaroslav, grand prince of Ruthenia, at Kyiv. While under the protection of the grand prince, Andronicus brought about an alliance between him and the emperor Manuel, and so restored himself to the emperor's favour. With a Russian army he joined Manuel in the invasion of Hungary and assisted at the siege of Semlin.

After a successful campaign Manuel and Andronicus returned together to Constantinople (1168); but a year after, Andronicus refused to take the oath of allegiance to the prince of Hungary, whom Manuel desired to become his successor. He was removed from court, but received the province of Cilicia.

Being still under the displeasure of the emperor, Andronicus fled to the court of Raymond, prince of Antioch. While residing here he captivated and seduced the beautiful daughter of the prince, Philippa, sister of the empress Maria. The anger of the emperor was again roused by this dishonour, and Andronicus was compelled to flee.

He took refuge with Amalric I of Jerusalem, whose favour he gained, and who invested him with the Lordship of Beirut. In Jerusalem he saw Theodora, the beautiful widow of the late King Baldwin III and niece of the emperor Manuel. Although Andronicus was at that time fifty-six years old, age had not diminished his charms, and Theodora became the next victim of his artful seduction. To avoid the vengeance of the Emperor, she fled with Andronicus to the court of the Sultan of Damascus; but not deeming themselves safe there, they continued their perilous journey through Persia and Turkestan, round the Caspian Sea and across Mount Caucasus, until at length they settled in the ancestral lands of the Comneni at Oinaion, on the shores of the Black Sea, between Trebizond and Sinope.

While Andronicus was on one of his incursions, his castle was surprised by the governor of Trebizond, and Theodora with her two children were captured and sent to Constantinople. To obtain their release Andronicus made abject submission to the Emperor and, appearing in chains before him, implored pardon. This he obtained, and was allowed to retire with Theodora into banishment at Oinaion.

In 1180 the emperor Manuel died, and was succeeded by his son Alexius II Comnenus, who was under the guardianship of the empress Maria. Her conduct excited popular indignation, and the consequent disorders, amounting almost to civil war, gave an opportunity to the ambition of Andronicus. He left his retirement, secured the support of the army and marched upon Constantinople, where his advent was stained by a cruel massacre of the Latin inhabitants, which was focused on the Venetian merchants who virtually controlled the economy of the city. Alexius was compelled to acknowledge him as colleague in the empire, but was soon put to death.

Andronicus, now (1183) sole emperor, married Agnes of France, widow of Alexius II and a child twelve years of age. Agnes was a daughter of Louis VII of France and his third wife Adèle of Champagne.

His short reign was characterized by strong and wise measures. He resolved to suppress many abuses, but, above all things, to check feudalism and limit the power of the nobles. The people, who felt the severity of his laws, at the same time acknowledged their justice, and found themselves protected from the rapacity of their superiors. The aristocrats, however, were infuriated against him, and summoned to their aid William II of Sicily. This prince landed in Epirus with a strong force, and marched as far as Thessalonica, which he took and destroyed; but he was shortly afterwards defeated, and compelled to return to Sicily.

Andronicus seems then to have resolved to exterminate the aristocracy, and his plans were nearly crowned with success. But in 1185, during his absence from the capital, his lieutenant ordered the arrest and execution of Isaac Angelus, a descendant of the first Alexius. Isaac escaped and took refuge in the church of Hagia Sophia. He appealed to the populace, and a tumult arose which spread rapidly over the whole city. When Andronicus arrived he found that his power was overthrown, and that Isaac had been proclaimed emperor. Isaac delivered him over to his enemies, and for three days he was exposed to their fury and resentment. His punishments included flogging, being tied to the back of a sick camel and having boiling water thrown in his face. At last they hung him up by the feet between two pillars. His dying agonies were shortened by an Italian soldier, who mercifully plunged a sword into his body. He died on September 12, 1185. Andronicus was the last of the Comneni to rule Constantinople, although his grandsons Alexius I of Trebizond and his brother David founded the Empire of Trebizond in 1204.