Alexios Branas was doubly linked to the imperial Komnenos family. He was the son of Michael Branas and of Maria Komnene, who was the great-niece of Alexios I Komnenos. He himself married Anna Vatatzaina, the niece of Manuel I Komnenos, and her sister, Theodora, was Manuel's lover.
Branas was one of relatively few generals who never rebelled against Andronikos I Komnenos, who rewarded his loyalty by raising him to the exalted rank of protosebastos. Branas led several successful campaigns on his behalf, against the forces of Béla III of Hungary in 1183, against a rebellion led by Andronikos, Isaac and Alexios Angelos in 1184, and against the Norman invaders under William II of Sicily in 1185 (Battle of Demetritzes).
In 1187, shortly after the accession of Isaac II Angelos, Branas was sent to counter the Vlach-Bulgarian Rebellion. This time he did rebel, but was defeated by Conrad of Montferrat, the emperor's brother-in-law, who commanded the centre of the imperial forces in the battle. Branas wounded Conrad, who nevertheless unhorsed him, his lance striking the cheekpiece of his helmet. Branas was then beheaded by Conrad's supporting footsoldiers. The head was taken to the imperial palace, where it was treated like a football, and was then sent to Branas's wife Anna, who (according to the historian Niketas Choniates) reacted bravely to the shocking sight.
It was probably after his death that his son, Theodore Branas, became the lover of the dowager Empress Anna (Agnes of France): they were together by 1193 according to the Western chronicler Alberic of Trois-Fontaines. Alexios Branas also had a daughter, probably named Eudokia, who married Isaac Angelos, son of the sebastocrator John Doukas.
- O city of Byzantium: annals of Niketas Choniates tr. Harry J. Magoulias (Detroit: Wayne State University Press, 1984) especially pp. 208–214.
- Kazhdan, Alexander, ed. (1991), Oxford Dictionary of Byzantium, Oxford University Press, p. 320, ISBN 978-0-19-504652-6
- Varzos, Konstantinos (1984). Η Γενεαλογία των Κομνηνών, Τόμος Β' [The Genealogy of the Komnenoi, Volume II] (in Greek). Thessaloniki: Byzantine Research Centre. pp. 394–417, 540–543.