An act establishing Mercer County took place on January 2, 1824. In 1837 Van Wert County was detached and the county line established is the current northern border of Mercer County. In 1839 Celina was established as the capital of Mercer County, St. Marys, Ohio was the previous capital. In 1848 the area south of the Greenville Treaty Line to the current southern county line, was attached.