The first traces of human occupation date back to the 3rd millennium BC and are concentrated in an easily defensible terrain at one of the bends of the Azat river. In the 8th century BC the area was conquered by the Urartian king Argishti I. The first literary testimony to the existence of a fortress on the spur crowning the site of Garni comes from the Roman historian Tacitus and dates from the middle of the 1st century AD.
Excavation of the existing remains was conducted for a brief period in 1909–1910 and was later resumed (1949) by Soviet archaeologists. The results have shown that the actual fortification had been erected much earlier, probably sometime in the 3rd century BC as a summer residence for the Armenian Orontid and Artaxiad royal dynasties. The fortress of Garni (Gorneas in Latin) became the last refuge of king Mithridates of Armenia, where he and his family were assassinated by his son in law and nephew Rhadamistus. Several constructions and buildings have been identified within the enclosed area, including a two-storey royal summer palace, a bath complex, a church built in AD 897, a cemetery and the site's most famous and best preserved edifice, a peristyle Greco-roman temple built in the Ionic order. Of particular interest is the bathhouse located in the northern part of the site.
It as a well preserved hypocaust and one of its floors is decorated with a mosaic reproducing a well known late Hellenistic iconographic type. It bore depictions of Greek mythological figures and personifications, such as Tethys, Oceanus, Thetis (Achilles's mother), Aigialos (literally sea-shore, spelled ΕΓΙΑΛΟΣ on the actual mosaic). The accompanying inscription, written in Koine Greek, ΜΗΔΕΝ ΛΑΒΟΝΤΕΣ ΗΡΓΑΣΑΜΕΘΑ (we worked without receiving anything) implies that the artists responsible for the construction of the mosaic received no fee for their labour.