what county is plymouth in
Sponsored Topics. Plymouth is a city in Marshall County, Indiana, United States. The population was 10,033 at the 2010 census. The city is the county seat of Marshall County. Plymouth was the site of the first retail outlet of defunct U.S. retailer Montgomery Ward in 1926. Plymouth, Indiana is located at 41Â°20â²38â³N 86Â°18â²45â³W / 41.34389Â°N 86.3125Â°W / 41.34389; -86.3125 (41.343894, -86.312544), along the Yellow River.