Optically transparent ceramics are essential materials for a variety of optical applications. In particular, MgO is of interest due to its wide transmission range (300 nm to 6 μm) and its excellent thermo-mechanical properties. Spark plasma sintering (SPS) has been the subject of recent studies dealing with transparent ceramics densification. SPS enables the application of very high heating and cooling rates, short dwell durations, and sintering at relatively low temperatures, which limits grain growth during densification. This study introduces exceptionally highly transparent MgO ceramics produced via SPS at relatively low temperature and pressure by optimal incorporation of LiF as a sintering additive. The effect of LiF content on the microstructural and optical properties is presented with emphasis on its function as a densification aid and an agent for minimizing residual carbon contamination. Fully dense MgO discs, 20 mm in diameter and 2 mm thick, with ∼80% in-line transmission at 800 nm and >85% transmission in the infrared range (2−6 μm), are attained. These results demonstrate outstanding transparency in polycrystalline MgO in the 800 nm range, only 7% below the theoretical value. In addition, this work strengthens our understanding of the LiF action mechanism during sintering: firstly, its role as a lubricant and consolidation aid in the liquid phase starting at 800 °C; secondly, its function as a gas-phase buffer minimizing inward diffusion of carbonaceous gasses following LiF evaporation around 1100 °C; thirdly, its effect on texture development in the SPS-pressing direction at 1250 °C. Our findings are in line with the theory that LiF acts as a physical buffer to lessen carbon penetration. Yet, to avoid excessive gas trapping and fracture development, the LiF concentration must be carefully regulated. Here, a concentration of 0.5wt.% is found to be optimal. These findings pave the way for fabrication of large, fully dense transparent MgO samples with nearly theoretical transparency.