He was born in the province of Asia, to a Scythian who had secretly converted to Christianity. Like his parents, he was a secret follower of Christ, serving as a legionary in the Roman army during the reign of the Emperor Decius. During a campaign, an Angel appeared to him, gave him a sword and told him to go into battle trusting in Christ's help. Mercurius plunged into battle, fought his way alone through the enemy lines, and reached the barbarian commander Rigas, whom he killed. Upon the death of their chief the barbarians scattered and the victory was won.

The Emperor, hearing of the young soldier's exploits, promoted him to a position at court. There, lulled by the pleasures and honors of the court, Mercurius forgot his duties to Christ his King. One night the same Angel who had given him the sword appeared to him once again and reminded him of the sword that Christ had given him, an emblem of the battle of martyrdom that he was about to enter. The next day Mercurius, now returned to his senses, refused to offer sacrifice to the gods. When called before the Emperor, he boldly proclaimed Christ and threw off his badges of office. He was thrown in prison and subjected to cruel tortures, all of which he bore with peace and joy, encouraged by the Angel who appeared to him again to offer comfort and encouragement. After long torment he was beheaded in Caesarea in Cappadocia, at the age of twenty-five.