This work focuses on the individual impact of nano- and ultrafine micro magnesia (N–MgO and UM-MgO, respectively) on the early performance of high volume fly ash (FA)-blended-cement mortar (CM). N–MgO and UM-MgO were synthesized using a thermal treatment of lightweight hydromagnesite at 550 and 1100 °C, respectively. Different contents of MgOs (1, 3, and 5 wt% by weight of powder) were individually added to FA-blended-CM. The results revealed that the N–MgO exhibits higher efficiency in the acceleration of early PC-FA system hydration compared to UM-MgO at all addition levels. The addition of 1, 3, and 5 wt% N–MgO to FA-blended-CM has resulted in the enhancement of 3-days compressive strength value by ~26, 94, and 103%, respectively; whereas the UM-MgO shows a modest effect. The small particle size, low crystallinity, and high hydration reactivity are the dominant features of N–MgO, which reflect on the enhancement of FA-pozzolanic activity accompanied by the acceleration of strength-giving-phases formation at early ages of hydration. To achieve the sustainability, we have recommend the use of N–MgO, as it contributes to cost minimization, energy saving, and the mitigation of carbon footprint.