After the Lombard conquest in 755 Terni lost prominence and was reduced to a secondary town in the Duchy of Spoleto. In 1174 it was destroyed by Frederick Barbarossa's army. In the following century Terni was one of the favourite seat of St. Francis' preaching.
In the 14th century Terni issued a constitution of its own and was the theater of inner disputes between Guelphs and Ghibellines, and later between the two parties of Nobili and Banderari, until it was included in the Church State.
In 1580 an ironwork, the Ferriera, was established to work the iron ore mined in Monteleone di Spoleto, starting the industrial tradition of the city. In the 19th century Terni took advantage of the Industrial Revolution and of the large presence of water sources in the area. New industries included a steelwork, a foundry, as well as weapons, jute and wool factories.
The presence of a strong industries concentration made it a favourite target for the Allied bombardments in World War II, totalling 108 raids.