Endolichenic fungi are diverse groups of predominantly filamentous fungi that reside asymptomatically in the interior of lichen thalli. Natural products from endolichenic fungi, isolated from a variety of different lichen species, have been attracting increased attention for their potential to produce bioactive metabolites possessing new structures and representing different structural classes. This is evident from the steady increase of publications devoted to endolichenic fungal metabolites over the past decade, since the first report of endolichenic secondary metabolites. The bioactive metabolites produced by endolichenic fungi originate from multiple biosynthetic pathways and occupy different chemical structure classes, including steroids, quinones, terpenoids, peptides, xanthones, sulfur-containing chromenones, etc. Endolichenic fungal metabolites possess a diverse array of bioactivities, such as anticancer, antiviral, antibacterial, antifungal, and anti-Alzheimer's disease. This review provides the first thorough assessment of endolichenic fungi, their biodiversity, secondary metabolites, and associated bioactivity. This review will highlight the bioactive metabolites reported in recent years from endolichenic fungi, as well as discussing the potential of these symbiotic fungi as sources of new, diverse natural products with varying bioactivities.