This research explores the potential of utilizing locally sourced fly ash as a sustainable alternative in structural concrete, with a primary focus on its impact on mechanical properties such as compressive and flexural strengths. Fly ash, a byproduct of coal combustion, offers an eco-friendly solution to reduce the environmental footprint associated with conventional concrete production. The fly ash used in this study was procured from Sarangani Energy Corporation, located in Maasim, Sarangani Province. Four different concrete mixtures incorporating fly ash were prepared and tested for both compressive and flexural strengths, each replicated five times to ensure statistical reliability, with samples air-cured at ambient temperature for 7, 14, 21, and 28 days. Response Surface Methodology (RSM) optimization was employed to identify the optimal conditions for achieving maximum compressive and flexural strengths in fly ash-based geopolymer concrete. The findings revealed that the optimal fly ash content for enhancing both compressive and flexural strengths is 20% of the cement composition, with a marked improvement in mechanical properties over ordinary Portland cement concrete (OPCC). This study underscores the importance of optimizing mix designs and curing conditions to maximize the benefits of fly ash incorporation. The research highlights the viability of fly ash as a sustainable and effective component in the production of structural concrete and recommends exploring additional properties such as shear strength, tensile strength, and permeability, as well as other influential factors like curing conditions and supplementary materials, to further substantiate the advantages of fly ash in concrete applications.