Diversity of tropical lichen-forming fungi, especially crustose lichens is currently poorly known. Since lichens are important bioindicators of air pollution, forest health, and climate change, we addressed the lichen diversity in Kenya. Our study focused on the diversity of lichen-forming fungi in the Mount Kenya montane forests, where we sampled corticolous lichens at ten localities in the Mount Kenya forest. The lichen diversity in the study area was very rich with fifty-nine species recorded for the first time from Kenya; 18 of them are new records for the African continent. Diagnostic features and distribution areas are given for each species reported. The crustose genera Auriculora, Candelariella, Clandestinotrema, Diorygma, Hemithecium, Lecanactis, Lepraria, Letrouitia, Megalospora, Mycoporum, Ocellularia, Placynthiella, Piccolia, Ramboldia, Tephromela and Thelotrema are reported here from Kenya for the first time. The majority of new records belong to pan- or palaeotropical species, in addition to cosmopolitan elements.