St. Elmo is a city in Fayette County, Illinois, United States. The population was 1,254 at the 2020 census. St. Elmo was established in 1871. St. …St. Elmo is a city in Fayette County, Illinois, United States. The population was 1,254 at the 2020 census. St. Elmo was established in 1871. St. Elmo is located at 39°1′36″N 88°51′7″W / 39.02667°N 88.85194°W.