Indian almond leaves (Terminalia catappa L.) are a good source of natural antioxidants, including flavonoids and polyphenols. This study utilized the antioxidant potential and application of Indian almond leaf extracts in a food system. Indian almond leaf powder was extracted using ethanol and water at a 1:10 ratio, then concentrated. The phytochemical contents, namely Total Phenolic Content (TPC) and Total Flavonoid Content (TFC), were analysed using the Folin–Ciocalteu reagent method and the aluminium chloride (AlCl₃) reagent method, respectively, whereas antioxidant activity was analysed using DPPH⁺ radical-scavenging activity. The extracts were then infused into coconut oil at different concentrations (200 ppm and 400 ppm, namely Sample 1 and Sample 2) and stored. After repetitive frying (with the same oil being reused for frying 4 times each day, at 6-minute intervals per session), oil stability was monitored by measuring peroxide value, free fatty acids, refractive index and thiobarbituric acid value (TBA). These were compared to the standard, synthetic antioxidant, Tert-butylhydroquinone (TBHQ). By day 3, the peroxide values were 4 meq/kg and 2 meq/kg, FFA values were 0.6768% and 0.5076% , RI values were 1.5617 and 1.5613, and TBA values were 0.035 and 0.0262 for the standard and samples (1 and 2), respectively. This study substantiates the efficacy of Terminalia catappa L. (Indian almond) leaf extract powder as a potent natural antioxidant, demonstrating its significant ability to enhance oil stability and inhibit oxidative degradation. The extract showed comparable, and in some cases superior, performance to conventional synthetic antioxidants, even at lower concentrations. These findings not only address the rising demand for clean-label and naturally derived food additives but also underscore the extract’s commercial viability due to its cost-effectiveness and ease of integration into existing food processing systems. In addition, its origin from plant-based waste material further adds to its sustainability profile, making it an attractive alternative in efforts to reduce dependency on synthetic preservatives.