Ionia is the largest city in, and the county seat of, Ionia County, Michigan, United States.The population was 11,394 at the 2010 census. Every late July (ending on the last Saturday of July and beginning 10 days previous) it hosts what is said to be the world's largest free-admission fair.efore Euro-American migration, this was the location of an Odawa village. The leader of the Odaw here was Cobmoosa. His people planted crops here in 1833 but when Dexter and his associates arrived, they sold these crops to Dexter and relocated to the Flat River.