Cordyceps militaris is one of the most popular mushrooms and nutraceuticals in Eastern Asia. This study assayed and compared the antimicrobial, antioxidant, and cytotoxic properties of the methanol extracts from fruiting bodies and fermented mycelia of C. militaris, as well as the contents of total phenol, flavonoids, and cordycepin. The results showed that the extracts from fruiting bodies possessed broad antimicrobial activities against all microorganisms tested (both bacteria and fungi), whereas that from the fermented mycelia showed selective activity. The antioxidant potential of two extracts is significant in the four tested systems in vitro, including total antioxidant capacity, scavenging abilities on 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radicals, reducing power, and chelating ability on ferrous ions. The fruiting bodies had stronger DPPH· radical scavenging activity, whereas the fermented mycelia had stronger total antioxidant capacity, chelating ability, and reducing power, which suggested that they had their own role and worked in different ways. Both extracts present strong activities against tumor cell line A549. The results obtained indicated that extracts from C. militaris might be valuable antimicrobial, antioxidant, and cytotoxic natural sources and seemed to be applicable in health and medicine as well as in the food industry.