These Algonquian speaking peoples of the Wabanaki Confederacy lived along what they called Newichwanook, now the Salmon Falls River.

Situated where the Salmon Falls River drops 100 feet (30 m) over a mile, Somersworth early became a mill town, beginning with gristmills and sawmills.

In 1822, the brothers Isaac and Jacob Wendell of Boston purchased for $5,000 a gristmill with its water rights at the Great Falls.

They established the Great Falls Manufacturing Company, a textile business that expanded to include three mills for spinning thread and weaving cotton and woolen fabrics, specializing in "drillings, shirtings and sheetings".

Throughout the 19th century, other expansive brick mill buildings, including a bleachery and dye works, were erected beside the river.

The building housed the operations that took the buff-colored fabric produced in the seven mills and transformed it into a sparkling white material that could be dyed or printed according to the buyer's wishes.

Brick tenement row houses were rented by the company to employee families, many of whose members worked in the mills beside their parents before passage of child labor laws.

In 1922, Somersworth was affected by the 1922 New England Textile Strike, shutting down the mills in the city over an attempted wage cut and hours increase.

Water power was replaced with newer forms of energy, and cotton could be manufactured where it grew, saving transportation costs.

Labor was also cheaper in the South, which did not have New Hampshire's inventory tax that levied commodities like coal and cotton at the plants.

Although frequently overshadowed by the larger neighboring cities of Dover and Rochester, Somersworth retains a quantity of Victorian architecture from its prosperous age.

The upper end of High Street, however, continues to develop as a retail center, with several big-box chain stores.

One ship of the United States Navy was named after the city; the USS Somersworth (PCE(R)-849) was a PCE-842-class patrol craft.

New Hampshire Route 108 passes through a western portion of the city, leading northwest to Rochester and south to Dover.

In August 2007, the upper floors of the school were closed by the New Hampshire State Fire Marshall after the city neglected to fix code violations they were warned about in November 2006.

On February 17, 2009, the Somersworth City Council voted to approve bonding in the amount of $19.9 million for the construction of a new elementary school.