Lake County is a county in the U.S. state of Ohio. As of the 2020 census, the population was 232,603. Its county seat is Painesville, and its largest city is Mentor. The county was established on March 6, 1840, from land given by Cuyahoga and Geauga counties. Its name is derived from its location on the southern shore of Lake Erie and the fact that the majority of the county’s land has always lied beneath Lake Erie. Lake County is part of the Cleveland, OH Metropolitan Statistical Area.