This study explores the mechanical properties of ultra-fine high volume fly ash (UFHVFA) mortar with the addition of slaked lime water. UFHVFA mortar incorporates a proportion of ultra-fine fly ash such as 50, 55, 60, 65, and 70% as a partial cement replacement. However, the slower pozzolanic reaction of fly ash often results in reduced early-age strength. This research examines how slaked lime water affects the key cementitious properties and of mechanical properties of UFHVFA mortar. Multiple mortar mix designs were prepared in the proportion of 1:3 ratio of cementitious material to natural sand, varying the ultra-fine fly ash replacement levels and slaked lime water concentrations as 1, 2, 3, 4, and 5 g per litre. Ultra-fine particle size of fly ash is kept as 3–5 µm. Cementitious pastes were tested for consistency, initial and final setting time. Mortar specimens were tested for compressive strength and flexural strength and durability for 7, 28, 56, and 90 days of curing. The outcomes demonstrate that when the ordinary Portland cement replaced up to 55% of UFHVFA, the compressive strength was significantly improved, while adding 4 g of slaked lime water concentration and maintaining the water cementitious powder as 0.3. As the result, there is an improvement in mechanical and durable properties. This work concludes the effect of lime water reacting with unreacted cement gel after the hydration of OPC particles over the long-term period, i.e. above 56 days, is unavoidable for strength enhancement.