Marion is a city in and the county seat of Grant County, Indiana, United States, along the Mississinewa River. The population was 28,310 as …Marion is a city in and the county seat of Grant County, Indiana, United States, along the Mississinewa River. The population was 28,310 as of the 2020 census. It is named for Francis Marion, a brigadier general from South Carolina in the American Revolutionary War.