Endophytic fungi provide valuable sources for the discovery of secondary metabolites that can be used as lead compounds in drug discovery. In this study, four new sesquiterpenoids with a farnesane backbone, schizophyllol A–B (1–2) and schizophylloside A–B (3–4), together with five known analogues (5–9), were isolated from the plant-derived fungus Schizophyllum sp. HM230. Their structures were established through extensive spectroscopic analyses including HR-ESI-MS and 1D and 2D NMR. The antioxidant activities of all the isolated compounds (compounds 1–9) were evaluated via hydroxyl radical scavenging, DPPH free radical scavenging, and superoxide anion radical scavenging assays. Compounds 1 and 2 displayed stronger antioxidant capacities than the positive control tert-butylhydroquinone. Furthermore, the antifungal activities of the isolated compounds were evaluated against four phytopathogenic fungi: Sclerotinia ginseng, Rhizoctonia solani, Cylindrocarpon destructans, and Exserohilum turcicum. All the test compounds demonstrated inhibitory effects; notably, compound 4 exhibited the strongest activities against the four selected phytopathogenic fungi, with inhibitory rates ranging from 42.3% to 65.4% at 0.2 mg/mL.