Mount Holly, Arkansas  Mount Holly is an unincorporated community in Union County, Arkansas, United States. Mount Holly is located at the junction of Arkansas highways 57 and 160, west-northwest of El Dorado. Mount Holly has a post office with ZIP code 71758. The Smackover-Norphlet School District (formerly the Smackover School District) operates public schools serving the community, including Smackover High School. It was previously served by the Mount Holly School District, which operated Mt. Holly Elementary School and Mount Holly High School. On July 1, 2004 the former Mt. Holly district was consolidated into the Smackover district. For a period the district continued operating Mt. Holly Elementary. The Smackover district asked for permission from the Arkansas Board of Education and received the permission in 2006.