Millfield is a census-designated place in northern Dover Township, Athens County, Ohio, United States. As of the 2010 census it had a population of 341. Although it is unincorporated, it has a post office, with the ZIP code of 45761. It is located near State Route 13 midway between Chauncey and Jacksonville. Route 13 formerly ran through the community, but a bypass relocated it close by.A post office was established at Millfield in 1827. Besides the post office, Millfield had a gristmill.The Millfield Mine disaster, the worst mine disaster in Ohio history, occurred at Millfield in 1930.