Lecanora markjohnstonii is described as new to science from the southeastern United States, with a primary center of distribution in the southern Appalachian Mountain region. This sterile, sorediate crust is saxicolous on both sandstone and granite and occurs commonly in mixed hardwood-conifer forests with rock outcrops. It is characterized by a gray-green, rimose-areolate thallus, erumpent, raised soralia, and the production of atranorin together with 2-0-methylperlatolic acid. Molecular phylogenetic analyses of newly generated rDNA assemblies from a broad sampling of lineages within the Lecanoromycetes and Arthoniomycetes inferred placement of the unknown crust in the Lecanoraceae, specifically within Lecanora. Analysis of the mtSSU gene region then inferred placement in the Lecanora subfusca group. Finally, a fully assembled and annotated mitochondrial genome was compared to other lichenized fungal mitogenomes, including the publicly available Lecanora strobilina mitogenome, and showed that the gene region atp9 was missing as in other members of the Lecanorales.