The site of present-day St. Catharines was settled by 3000 United Empire Loyalists at the end of the 18th century. As farmers, the Loyalists found the area's open fields with fertile soils and permanent streams an attractive location for settlement. From 1790, the settlement (then known as "The Twelve") grew as an agricultural community and was also known for its services to travellers along the native trails at the foot of the escarpment.
St. Catharines was once referred to Shipman's Corners after Paul Shipman, owner of a tavern that was an important stagecoach transfer point. Increasing amounts of grain and lumber produced by settlers in the surrounding area led to the establishment of many saw and gristmills along Twelve Mile Creek. Although the War of 1812 saw widespread destruction along the Niagara frontier, Shipman's Corners survived due to its inland location. Its mills were in great demand as settlers were reluctant to rebuild closer to the Canadian - American border.
In 1815, leading businessman William Hamilton Merritt abandoned his wharf at Queenston near the border and set up another at Shipman's Corners. He became involved in the construction and operation of several lumber and gristmills along Twelve Mile Creek. Shipman's Corners soon became the principal milling site of the eastern Niagara Peninsula. At about the same time, Merritt began to develop the salt springs that were discovered along the river which subsequently gave the village a reputation as a health resort.
By this time St. Catharines was the official name of the village; the origin of the name remains obscure, but is thought to be named after Catharine Askin Robertson Hamilton, wife of the Hon. Robert Hamilton, a prominent businessman.
In 1825, the Erie Canal was completed, linking Lake Erie with the Hudson River and the Atlantic Ocean. Canadians decided that efforts were needed to move Canadian exports wholly through Canada rather than remaining dependent upon access via the Erie Canal through the United States. Merritt devised a canal scheme from Lake Erie to Lake Ontario that would provide a more reliable water supply for the mills while at the same time function as a canal. He formed the Welland Canal Company, and construction took place from 1824 to 1829. The canal and the mills made St. Catharines the most important industrial centre in Niagara. The Welland Canal became essential for ship building and St. Catharines prospered from the presence of the canal, while areas away from the route of the canal stagnated or declined.
By 1845, St. Catharines was incorporated as a town, with the town limits extending in 1854. By this time, railway traffic connected St. Catharines south to Port Colborne and west to Hamilton. Administrative and political functions were added to St. Catharines in 1862 when it became the county seat of Lincoln.
During this time, the 40 original wooden locks of the Welland Canal were replaced by 27 stone locks that were longer, wider and deeper. This canal was commonly known as the Second Welland Canal, although it followed a nearly identical course as the original canal. In time, milling declined as the newer vessels found the canal too shallow to navigate with heavy loads of grain. As a result, grain was lightered (transported) by rail to mills in Port Colborne and Port Dalhousie. St. Catharines farmers could not compete with the low-cost wheat from other places and were forced to look for other products. Some found the soils and climate of the lake plain to the north of the Niagara Escarpment were suited to fruit cultivation, and peaches became a major crop.
By the middle of the 19th century, St. Catharines was developing an international reputation as a health spa and was patronized by the elite in Canada as well as wealthy southern gentlemen and their families. At the same time, the passing of the American Fugitive Slave Act in 1850 meant that many freedom seekers, both slaves and non-slaves, looked north to find a country that granted them freedom and equality under the law. Harriet Tubman, the famous conductor of the Underground Railroad, used St. Catharines as her home base for several years in the 1850s. The freedom seekers lived, worked and built a new life here in St. Catharines with the support of religious and political leaders like William Hamilton Merritt, Hiram Wilson and Harriet Tubman herself. The British Methodist Episcopal Church, Salem Chapel on Geneva Street, a designated national historic site, serves as a place of pilgrimage for many today who wish to walk in the footsteps of the great abolitionists of that time, such as Frederick Douglass and John Brown.
In 1871, construction began on the third Welland Canal, which attracted additional population to the town. As a consequence of continual growth, the town limits were again extended. St. Catharines attained city status in 1876 with its larger population and area. By 1881, there were more acres in urban use in St. Catharines than there were rural acres in use. The completion of the third Welland Canal in 1887 however led to an eventual population decline due to the departure of large numbers of canal workers. The shipyards and mills had relocated as well.
Manufacturing became increasingly important in St. Catharines in the early 1900s with the abundance of hydro-electric power, its location on important land and water routes, and the imposition of industry tariffs. The large increase in population after the 1900s was mainly due to the continued industrialization and urbanization of the northern part of the city and the related expansion of business activity.
Two bridges were constructed in 1915, which solved the problem of access to the west and south sections of the city resulting in the number of houses doubling in the western area. The fourth Welland Canal was opened in 1932 as the third canal could no longer accommodate the larger ships.
The post war years and the automobile brought great change to the urban form of St. Catharines as suburbs spread and the general standard of living improved for many people. St. Catharines began to spread its boundaries in all directions with land being added five times during the 1950s. The Town of Merritton, Village of Port Dalhousie and Grantham Township were all incorporated as part of St. Catharines in 1961. In 1970 the Province of Ontario implemented a regional approach to deal with such issues as planning, pollution, transportation and services. As a result, Louth Township on the west side of the city was amalgamated, extending the city's boundary to Fifteen Mile Creek.