Somersworth had its beginning in the history of Dover as the land of the City was originally within Dover's boundaries. As the population grew, the community recognized the lure and convenience of the Salmon Falls River and no longer wished to travel to the Cocheco River and the distant church. In 1729 a portion of the community separated into their own "summer" parish and used a barn for a meetinghouse.

By 1754, this parish, which is now the area encompassed by both Rollinsford and Somersworth, had grown to be able to petition the State to become a town. As settlements moved up the Salmon Falls River to find more land and water power, its history was duplicated in the area known as Great Falls. Early settlers, with the names of Wentworth and Horn, built sawmills and gristmills.

In 1823, Gershom Horn's gristmill, water power, and land by the Great Falls on the Salmon Falls River sold for $5,000. This became the No. 1 mill of the Great Falls Manufacturing Company, which eventually grew into three mills, a bleachery, a first-in-the-state gasworks, and warehouses. The rural nature of the community changed and its development mirrored that of many other New England mill towns. Waves of immigrants, beginning with Irish, then French-Canadian, and finally Greeks, all left their distinct mark.

The railroad and a freight station came in 1842 and by 1887, a new passenger station and a roundhouse were developed. Other industries and businesses followed the growth, including the Somersworth Machine Company, whose "White Mountain" stove was distributed nation-wide, a woolen mill, and light and power company. Downtown boasted two banks, two hotels, and numerous stores selling goods to all those employed. By 1893, the community incorporated as a city.

The Great Depression closed many of these manufacturing companies and significantly impacted the areas prosperity until after World War II when General Electric expanded its meter manufacturing operation to the newest of the abandoned cotton mills. Urban renewal of the 1960's forever changed the appearance of Main Street with the removal of brick and wood-framed tenement mill housing, an opera house, and several commercial blocks. However, the Market and High Street areas still retained many buildings from the nineteenth century, and some have been restored to their original appearance. The Somersworth Housing Authority refurbished mill housing on Market Street, and renovated a former shoe and textile mill for elderly housing.

A strong economy in the late 1980's and again in the late 1990's brought diversity to the City's economic base, both in outlying commercial and industrial areas as well as the Downtown on Market, Main and High Streets. The remaining original brick cotton factory buildings have been renovated by the Great Falls Mill Limited Partnership and currently house small manufacturing and service companies including children's games and furniture, printing, engineering services and a fitness center. Although the names have changed throughout the last 170 years, downtown is still a blend of offices, retail shops, markets, restaurants, places of worship and meeting, homes, and apartments.

More recent changes include a new City Hall in the heart of downtown, façade rehabilitation at the Somersworth Plaza and streetscape improvements.