Mahoning County is a county in the U.S. state of Ohio. As of the 2020 census, the population was 228,614. Its county seat and largest city is Youngstown. The county is named after the Mahoning River and was formed on March 1, 1846; the 83rd county in Ohio. Until 1846, the area that is now Mahoning County was part of Trumbull and Columbiana counties, when the counties in the area were redefined and Mahoning County emerged as a new county.