It was long recognized that St. Louis sits at the geographic center of Michigan's lower peninsula "as closely as can be determined." A campaign by Clarence and Odessa Smazel in the early 1950's, owners of the St. Louis Leader-Press, a now-defunct local newspaper, resulted in the State of Michigan designating it as such: a sign in Clapp Park on Highway M-46 denotes the city as the official "Middle of the Mitten." One of the enjoyable results of this designation is the apparently irresistible urge to use the Middle Of the Mitten acronym on nearly any type of business venture. However, an unexplained mystery is exactly how the first letter of the article "of" is capitalized, and the first letter of the "the" is not.
St. Louis is the site of the former Michigan Chemical Corporation plant that helped commercialize DDT
After its purchase by Velsicol Chemical Corporation, the plant was responsible for a product mixup which resulted in contamination of cattle feed throughout Michigan and the Midwest in the 1970s.
It was the understanding of the citizens of St. Louis that the mixup occurred due to a "triple witching hour" of events: a labor strike with management replacing workers, a shortage of bags that helped distinguish between cattle feed additive and the flame retardant PBB, and storing both the cattle feed and the fire retardant in the same dimly lit warehouse.
St. Louis has made a bit of an economic recovery since by inviting the State of Michigan to build a minimum-security prison between Union and Croswell Roads, on the former Reichard "Michigolden" duck hatchery property. 
In the late nineteenth century, St. Louis was a destination for people seeking the health benefits of local mineral-rich water. The Park Hotel became noted as a health spa, and for its duck dinners as well.
Dredging has been underway for years to remove contaminants from the Pine River which runs alongside the property where the plant was located. The plant has been disassembled for a number of years and the land remains vacant to this day.
There were 1,468 households out of which 31.5% had children under the age of 18 living with them, 47.6% were married couples living together, 13.4% had a female householder with no husband present, and 34.3% were non-families. 29.5% of all households were made up of individuals and 12.6% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 2.92.
In the city the population was spread out with 20.6% under the age of 18, 12.1% from 18 to 24, 35.0% from 25 to 44, 20.9% from 45 to 64, and 11.4% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 142.5 males. For every 100 females age 18 and over, there were 155.6 males.
The median income for a household in the city was $31,122, and the median income for a family was $39,271. Males had a median income of $29,028 versus $19,272 for females. The per capita income for the city was $14,196. About 10.1% of families and 15.9% of the population were below the poverty line, including 19.9% of those under age 18 and 10.7% of those age 65 or over.