He was born in Illyricum and named Caesar (Junior Emperor). When his father died in 337 AD he was the leading individual in the massacre of family members. He ruled the Eastern section of the Empire but when Constantine II died trying to overthrow Constans in Italy he turned his attention to the West.
Constantine II eventually met and crushed Magnentius in battle at Mursa Major, one of the bloodiest battles in Roman history, in 351 AD. Magnentius committed suicide in 353 AD leaving Constantine II sole master of the Roman Empire.