Isabella County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 70,311. Its county seat is Mt. Pleasant. The area was known as Ojibiway Besse, meaning "the place of the Chippewa". It was set off as a county in 1831, and county government organized in 1859. It was named after Queen Isabella I of Castile, based on a suggestion by Henry Rowe Schoolcraft, US Indian Agent and ethnologist.