The Mahoning River watershed is located in northeastern Ohio. It drains a total of 1,085 square miles in Ohio and flows through all or part of seven counties. Major municipalities partially or fully in the watershed include Youngstown, Warren, Alliance, and Lordstown.

The eastern portion of the watershed is predominantly comprised of urban development with some forest and agricultural lands. The western portion of the watershed is a mixture of forest, hay, and pasture lands, cultivated crops, and urban development.