The first architectural phase of the theatre is dated to the early Hellenistic period, as is suggested by the figurines and architectural parts which were found either in situ or as spolia used in the masonry of the castle. The theatre was remodelled in the late Roman period. The horseshoe-shaped cavea would have a capacity of about 10.000 people. Plutarch, in the life of Pompeius (Pompeius, 42) informs us that the Roman general admired the theatre of Mytilene and reproduced its plan in the theatre that he built in Rome in 55 B.C.
The first excavations on the site were carried out in 1928 by D. Evangelides, who resumed the investigation in 1958, especially in the area of the orchestra and the skene. The site was cleared and drawn by Vassileios Petrakos in 1967.
In 1968, the Ephor of Antiquities V. Petrakos undertook the reconstruction of the peripheral wall of the orchestra and the restoration of the two rooms to the west of the orchestra.
The last architectural phase of the theatre, dated to the Roman period, is the one seen today. The circular orchestra has a diameter of ca. 24.20 m. and is separated from the cavea, now completely ruined, by a wall with marble orthostates. The skene is divided by three corridors; in the middle corridor a built drainage channel was uncovered. Two rooms, quarried out in the rock of the cavea, were found to the west of the orchestra.