Magnesium oxide, generally applied as a filler in high-temperature cells (with an electrolyte melting point above 250 °C), was modified with magnesium fluoride to improve its mechanical and electrical properties. Samples containing 10 and 25 mol.% MgF2 were prepared and calcined at 500, 600, and 700 °C. They were characterized by low-temperature nitrogen adsorption and X-ray diffractometry (XRD). Moreover, the electrolyte absorption, mechanical strength of pellets made of filler and electrolyte, and volume of unfilled spaces were determined. It was shown that the introduction of MgF2 in the amount of 10 and 25 mol.% results in a considerable decrease in the surface area of the initial MgO, which testifies to the covering of MgO by the formed fluoride. However, no new crystalline phases were formed as concluded from the XRD analysis. The pellets consisting of electrolyte and MgF2/MgO filler (the electrolyte + 40 wt.% of the filler) had a higher mechanical strength compared to bare MgO filler. In particular, they outperformed MgO in the ionic conductivity of molten electrolyte. The latter was almost three times as high as that of MgO filler, when the filler containing 25 mol.% MgF2 was employed. The aforementioned properties of MgF2/MgO materials predispose them for use as fillers in high-temperature cells.