Marion is a city in Linn County, Iowa, United States. The population was 26,294 at the 2000 census and was 41,535 in 2020, an increase of 5…Marion is a city in Linn County, Iowa, United States. The population was 26,294 at the 2000 census and was 41,535 in 2020, an increase of 58%. The city is located next to Cedar Rapids and part of the Cedar Rapids Metropolitan Statistical Area.