Well cementing plays a critical role in hydrocarbon exploration by ensuring zonal isolation, casing support, and long-term well integrity. Under elevated temperature conditions relevant to oil well environments, conventional cement-based materials may experience strength reduction and microstructural degradation, potentially leading to compromised structural performance. This study investigates the use of Banana Leaves Ash (BLA), an eco-friendly agricultural byproduct, as a potential supplementary cementitious material in cement mortar systems exposed to high-temperature conditions. Cement mortar formulations containing 0%, 5%, 10%, and 15% BLA by weight of cement were prepared and cured for 7, 14, and 28 days. Mechanical performance was evaluated through compressive, tensile, and flexural strength tests, while thermal stability was assessed after controlled heating at 50 °C, 200 °C, 300 °C, and 400 °C. Results indicate that BLA incorporation enhances the mechanical performance of the cement mortar, with the 15% BLA mixture exhibiting the highest strength values. After 28 days of curing, compressive strength reached a maximum of 27.726 MPa at 300 °C, representing a 16.75% improvement compared to the control sample. Flexural and tensile strengths increased by 18.77% and 7.39%, respectively. These findings suggest that Banana Leaves Ash has potential as a sustainable supplementary material for cement-based systems subjected to elevated temperatures, contributing to both improved material performance and the valorization of agricultural waste.