Clark County is a county located in the west central portion of the U.S. state of Ohio. As of the 2020 census, the population was 136,001. Its county seat and largest city is Springfield. The county was created on March 1, 1818, and was named for General George Rogers Clark, a hero of the American Revolution. Clark County comprises the Springfield, OH Metropolitan Statistical Area, which is also included in the Dayton-Springfield-Sidney-OH Combined Statistical Area.