St. Thomas Assembly was an automobile plant located in Southwold, Ontario, Canada, close to the Talbotville community and the nearby city of St. Thomas. The 2,600,000 sq ft (240,000 m2) facility, situated on a 635 acres (2.57 km2) site, opened in 1967, building the Ford Falcon. Flexible fuel vehicles (FFV) capable of operating on ethanol fuel were manufactured there during the later years of the assembly plant. Ford's plans for sustainability and reduction of fossil fuel consumption relied on the St. Thomas Plant and its Lincoln Town Car vehicles for years. It also produced the final Mercury vehicle, a Mercury Grand Marquis, after Ford decided to discontinue the Mercury brand after the 2011 model year.

Southwold is a township in Elgin County, in Ontario, Canada, located on the north shore of Lake Erie. It is a rich agricultural zone producing predominantly corn and soybeans. It is part of the London census metropolitan area.