Mercer is a city in Mercer County, Missouri, United States. The population was 263 at the 2020 census. Mercer was platted in the 1870s when t…Mercer is a city in Mercer County, Missouri, United States. The population was 263 at the 2020 census. Mercer was platted in the 1870s when the railroad was extended to that point. The community has the name of Marion Merwitt, the daughter of a local pharmacist.