Fly ash is a waste product from coal combustion and can be used as a partial substitute for cement, thereby aiding in the production of lower-carbon-footprint concrete. Most studies have focused only on mechanical or environmental performance, and fewer studies are available that integrate properties of fly ash concrete in terms of mechanical performance, water absorption and CO₂ emissions at low replacement rates (£20%). Accordingly, this study aims to meet this need by developing an integrated eco-mechanical evaluation to identify a practical replacement criterion based on three indicators. Five mixtures with varying amounts of fly ash (0%, 5%, 10%, 15% and 20%) were developed. The compressive strength, splitting tensile strength and water absorption after 28 days were tested in the laboratory, while the cradle-to-gate CO₂ emission was calculated by calculating the CO₂ emission from every constituent of the concrete. These results indicate that the 5% fly ash was able to achieve about 94.3% compressive strength of the control mixture, while also reducing CO₂ emissions by 4.71%, yielding the best strength–carbon balance of the mixtures. Increased replacement levels resulted in higher emission reduction, up to 18.85% at 20% fly ash, while producing more significant strength reductions. The findings serve as the basis for policy and specific specifications that would be associated with appropriate utilization of locally available fly ash in accordance with strength and durability requirements. It could be encouraged through material availability, transport, processing, quality control and the possibility of cement savings at the project level, through financial incentives or procurement preferences and performance-based standards.