Niles is a city in Trumbull County, Ohio, United States. The population was 18,443 at the 2020 census. Located at the confluence of the Mahonin…Niles is a city in Trumbull County, Ohio, United States. The population was 18,443 at the 2020 census. Located at the confluence of the Mahoning River and Mosquito Creek, Niles is a suburb in the Youngstown–Warren metropolitan area.