Germantown is a city in Montgomery County, Ohio, United States. The population was 5,796 at the 2020 census. A part of the Dayton metro…Germantown is a city in Montgomery County, Ohio, United States. The population was 5,796 at the 2020 census. A part of the Dayton metropolitan area, Germantown was founded by German Americans from Pennsylvania and was once home to a cigar industry.