The mangrove fungi provide a vast and unexplored source of diverse and unique chemicals and biological properties. The plant Aegiceras corniculatum (L.) Blanco and its endophytic fungus aspergillus species were collected from different sites of the Baleswar river region in Sundarban. Hence, we compared the antioxidant properties of the associated fungus ACSF-1 and the methanolic bark extract of Aegiceras corniculatum (MBAC) by measuring the total phenolic content (TPC), total flavonoid content (TFC), and DPPH free radical assay. Subsequently, antimicrobial activity was measured using the disc diffusion method, and cytotoxic activity was measured using the brine shrimp lethality bioassay. The results showed that MBAC has even more DPPH scavenging activity (IC50 = 44.036 μg/mL), TPC (310.275 mg GAE/g), and TFC (66.275 mg QE/g) in comparison with DPPH scavenging activity (IC50 = 92.542 μg/mL), TPC (234.832 mg GAE/g), and TFC (134.887 mg QE/g) in ACSF-1. The median lethal concentration value (LC50) of MBAC and ACSF-1 was found to be 43.93 μg/mL and 336.84 μg/mL, respectively. Moreover, MBAC showed a dose-dependent antimicrobial response to Escherichia coli and Staphylococcus aureus, whereas ACSF-1 was found to have activity against Bacillus subtilis and S. aureus. These results emphasize the unique pharmacological characteristics of both the plant and fungus, indicating their potential usefulness in various therapeutic fields.