[1][2] He was born around 270 AD in Sirmium, present-day Sremska Mitrovica, as the son of a military officer.

He was sentenced to death for preserving Christian texts and executed on April 9, 304, by order of Governor Probus.

Although the exact manner of his martyrdom remains uncertain (whether by sword or spear), it is known that the execution took place one mile east of Sirmium's city walls, on the road to Bassianae.

St. Demetrius is also believed to have been a co-patron saint of the small Romanesque chapel near the entrance to the city's imperial palace.

According to later Thessalonikian tradition, he is wrongly portrayed as a young legionary, vested in Roman armour.