Isaria cicadae Miquel (IcM) is an edible fungus used in traditional Chinese medicine. In this study, we identified the nutritional components of IcM using classical analytical methods and investigated the protective effects and mechanisms of intestinal mucosal inflammation mitigation and repair capacity in a dextran sulfate sodium (DSS)-induced ulcerative colitis (UC) mouse model. IcM, mesalazine alone, and co-administration significantly reduced colonic pathological damage, downregulated inflammatory factors (anti-tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), high mobility group box 1 (HMGB1), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), secretory immunoglobulin A (SIgA)), and improved the severity of UC in a dose-dependent manner by regulating the interaction between glucocoriticoid receptor (GR) and key proteins in the Hippo pathway, remodeling the intestinal mucosal barrier tissue structure by increasing the expression of mucin 2 (MUC2), Occludin, zonula occluden-1 (ZO-1), and E-cadherin, whereas high-dose IcM treatment increased significantly. Moreover, IcM and mesalazine have a synergistic effect, weaken toxicity to the liver and kidney, and contribute to alleviating the disease process in clinical patients.