The city, located at the intersection of two historical roads, was first settled in 1810. It was named the seat of the new Elgin County in 1844 and was incorporated as a village in 1852, as a town in 1861. In 1871, St. Thomas and the nearby village of Millersburg (a village east of the town) amalgamated. In 1881 St. Thomas finally grew to become a city.
Life-sized Jumbo statue
Ten years after the incorporation as a town, the developing village of Millersburg, which included these lands east of the London and Port Stanley Railway, amalgamated with St. Thomas.
In the late 19th century and early 20th century several railways were constructed through the city, and St. Thomas became an important railway junction. A total of 26 railways have passed through the city since the first railway was completed in 1856. In the 1950s and 1960s, with the decline of the railway as a mode of transportation, other industry began to locate in the city, principally primary and secondary automotive manufacturing.