Objectives: Plant-associated fungi are a biodiversity-rich group of microorganisms that are normally found within plant tissues or in the intercellular spaces. Endophytic fungi are capable of producing important “phytochemicals" that displayed variable therapeutic activities. The current study aimed to isolate the endophytic fungi associated with Callistemon citrinus (L.) stem tissues and evaluate their potential therapeutic activity. C. citrinus is a well-known plant of family Myrtaceae, known as the 'red bottlebrush. Methods: The isolated endophytic fungus was characterized phenotypically and genotypically, subjected to large-scale fermentation and extracted with ethyl acetate to obtain a crude extract.  The antimicrobial activity of the pure isolated compound was evaluated against Propionibacterium acne ATCC 6919 using agar well-diffusion assay. The MTT assay was utilized to evaluate the cytotoxicity of the crude extract against MCF-7 (human breast adenocarcinoma) cells.Apoptotic and necrotic investigation were applied. Results: Aspergillus flavus AUMC 16367 was isolated, characterized phenotypically and genotypically, and subjected to large-scale fermentation. Kojic acid, a well-known bioactive compound, was the major metabolite isolated and identified from A. flavus extract. A. flavus crude extract displayed strong cytotoxic activity against MCF-7 cell line showing IC₅₀ value of 15.89±0.33 µg/ml and apoptotic and necrotic effects. Kojic acid exhibited no antimicrobial activity against Propionibacterium acnes ATCC 6919.  Conclusion: Kojic acid was isolated from the fungal extract in significant amount constituting nearly 67% of the total weight of the crude extract. A. flavus crude extract displayed significant cytotoxic effects against MCF-7 and highlighting the potential therapeutic applications of this natural extract.