Mecosta County is a county located in the U.S. state of Michigan. As of the 2020 Census, the population was 39,714. The county seat is Big Rapids. The county is named after Chief Mecosta, the leader of the Potawatomi Native American tribe who once traveled the local waterways in search of fish and game. Chief Mecosta was one of the signers the Treaty of Washington in 1836. The easily navigated waterways soon led to a boom in lumber industry growth.