Ionia County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 63,905. The county seat is Ionia. The Ionia County Courthouse was built by Claire Allen, a prominent southern Michigan architect.Ionia County comprises the Ionia, MI Micropolitan Statistical Area, which is also included in the Grand Rapids-Wyoming-Muskegon, MI Combined Statistical Area.