Constans was born in about AD 320, as the son of Constantine and Fausta. He was educated at Constantinople and was proclaimed Caesar (junior emperor) in AD 333.
In AD 337 Constantine died and Constans became joint emperor with his two brothers, Constantine II and Constantius II, after they had agreed to execute the other two heirs and nephews of Constantine, Dalmatius and Hannibalianus.
As the relationship with Constantine II worsened, Constans became ever more reluctant to accept his elder brother as senior Augustus.
Finally in AD 340 things between Constantine II and Constans reached crisis point. Constans was at the Danube dealing with the suppression of the Danubian tribes. Constantine II took this opportunity to launch an attack on Italy.
Though the joint rule of the two brothers was not an easy one. Had the 'Nicene Creed under their father Constantine defined the Christian branch of Arianism as heresy, then Constantius II was effectively a follower of this form of Christianity, whereas Constans oppressed it in accordance to his father's wishes.
In his role as a Christian emperor, much like his father Constantine, Constans took an active part in trying to promote Christianity. In turn this led him to continue the persecution of the Donatist Christians in Africa, as well as to act against the pagans and the Jews.
In AD 341/42 Constans gained notable victories against the Franks and along the Danube, before crossing to Britain where he oversaw operations along Hadrian's Wall.
But Constans was an unpopular ruler, especially with the troops. So much so, they overthrew him. In January AD 350 a mutiny was led by Magnentius, a former slave of Constantine who had become Constans' army chief. The mutineer proclaimed himself Augustus at Augustodunum (Autun) and Constans was forced to flee toward Spain. But one of the usurper's agents, a man named Gaiso, caught up with Constans on the way and killed him.