In 337 he succeeded his father, jointly with his older brothers Constantine II and Constantius II, receiving Italy, Pannonia and Africa as his portion. Constantine II, who ruled over Gaul, Spain and Britain, attempted to take advantage of his youth and inexperience by invading Italy in 340, but Constans defeated Constantine at Aquileia, where the older brother died. The invasion was the effect of brotherly tensions between the two emperors. Constantine II was, at first, Constans's guardian. As Constans grew older, Constantine II never relinquished that position.
In 341-2, Constans led a successful campaign against the Franks and in the early months of 343 visited Britain. The source for this visit, Julius Firmicus Maternus, does not give a reason for this but the quick movement and the danger involved in crossing the channel in the dangerous winter months, suggests it was in response to a military emergency of some kind, possibly to repel the Picts and Scots.
Regarding religion, Constans was tolerant of Judaism but promulgated an edict banning pagan sacrifices in 341. He suppressed Donatism in Africa and supported Nicene orthodoxy against Arianism, which was championed by his brother Constantius the latter. Constans called the Council of Sardica, which unsuccessfully tried to settle the conflict.
In 350, the general Magnentius declared himself emperor with the support of the troops on the Rhine frontier, and later the entire Western portion of the Roman Empire. Constans lacked any support beyond his immediate household, and was forced to flee for his life. Magnentius' supporters cornered him in a fortification in Helena, southwestern Gaul, where he was killed by Magnentius's assassins.