As of the census of 2010, there were 11,766 people, 4,862 households, and 3,044 families residing in the city.There were 5,199 housing units, of which 337, or 6.5%, were vacant.
The Great Depression sent many regional textile firms into bankruptcy, when some local facilities were adapted for shoemaking.
The railroad arrived in the early 1840s, before which goods were carted to Dover.
At first millworkers came from surrounding farms, including those in Berwick, Maine, directly across the bridge. But as the need for labor grew, immigrants arrived from Ireland, and later Quebec.
Somersworth, originally called "Sligo" after Sligo in Ireland, was settled before 1700 as a part of Dover.
It was organized in 1729 as the parish of "Summersworth", meaning "summer town", because during that season the ministers would preach here.