Smithton is a city in Pettis County, Missouri, United States. The population was 570 at the 2010 census. Smithton was platted in 1859. It was na…Smithton is a city in Pettis County, Missouri, United States. The population was 570 at the 2010 census. Smithton was platted in 1859. It was named for Gen. George R. Smith, a railroad promoter, or for General Thomas A. Smith.