Northfield is a home rule-class city in Jefferson County, Kentucky, United States. The population was 1,020 at the 2010 census. Northfield wa…Northfield is a home rule-class city in Jefferson County, Kentucky, United States. The population was 1,020 at the 2010 census. Northfield was incorporated in July 1965. One of the factors in incorporation was the threat of annexation by nearby St. Matthews. The city is located on a subdivision of the former 3,500 acre Glenview horse farm.