This study explores the application of wood ash (WA) as a partial replacement for PC in mortar. Three pre-treatment methods were applied to WA to enhance its reactivity, and it was then incorporated into mortar at two different substitution levels of 10 and 30%. Tests on compressive and flexural strength were conducted on the hardened mortar samples. All hardened mortar samples containing WA showed a decrease in mechanical properties compared to the reference sample without WA. The highest compressive and flexural strength of the samples with WA were observed for those containing 10% of sieved and slaked WA. The compressive and flexural strength of these samples after 28 days were 56 and 9 MPa, respectively, whereas those of the reference samples were 62 and 10 MPa, re-spectively. Based on the results, the best-performing samples on the compressive test un-derwent additional testing for freeze–thaw resistance to assess their durability. The mass loss of the reference sample and that with 10% of sieved and slaked WA after 56 freeze–thaw cycles was 11,800 and 13,800 g/m2, respectively. The findings revealed that increas-ing the proportion of WA typically led to a decline in the mechanical properties of mortar compared to conventional mixtures. However, with appropriate pre-treatment techniques, the quality and performance of mortar containing WA were significantly improved, demonstrating its potential as a sustainable alternative in reducing the carbon footprint of PC production.