St. Thomas is a city in Cole County, Missouri, United States. The population was 222 at the 2020 census. It is part of the Jefferson City…St. Thomas is a city in Cole County, Missouri, United States. The population was 222 at the 2020 census. It is part of the Jefferson City, Missouri Metropolitan Statistical Area. A post office called Saint Thomas has been in operation since 1855. The city was named after Thomas the Apostle.