According to the early medieval Georgian annals and hagiography, Mirian was the first Christian king of Iberia, converted through the ministry of Nino, a Cappadocian female missionary.

[16] After Mirian's marriage with Abeshura (daughter of the previous Iberian ruler Aspacures), 40,000 Sasanian "select mounted warriors/cavalry" were subsequently stationed in eastern Iberia, Caucasian Albania and Gugark.

[18] Indeed, according to the modern historian Stephen H. Rapp, the Caucasus was part of the "Iranian Commonwealth", "a massive cross-cultural enterprise stretching from Central Asia to the Balkans.

[3] Nevertheless, as Ammianus Marcellinus recounts, Constantine's successor, Constantius, had to send in 360 embassies with costly presents to Arsaces of Armenia and Meribanes of Iberia to secure their allegiance during the confrontation with Iran.

A legend has it that when Mirian, staunchly pagan, was hunting in the woods near his capital Mtskheta, the darkness fell upon the land and the king was totally blinded.

[5][26] His conversion fostered the growth of the central royal government, which confiscated the pagan temple properties and gave them to the nobles and the church; the medieval Georgian sources give evidence of how actively the monarchy and the nobility propagated Christianity and of the resistance they encountered from the mountain folk.

[27] The Roman historian Rufinus as well as the Georgian annals report that, after their conversion, the Iberians requested clergy from the emperor Constantine, who responded vigorously and sent priests and holy relics to Iberia.

The Georgian tradition then relates a story of the construction of a cathedral in Mtskheta at Mirian's behest and the king's pilgrimage to Jerusalem shortly before his death.

His first wife was Abeshura, daughter of the last Arsacid Iberian king who also traced his ancestry to the ancient Pharnabazid dynasty of Iberia.