MgO-modified mesoporous silicas (MgO/SBA-15) were prepared in a one-pot synthesis method. In the present study, a silica precursor was rapidly hydrolyzed and condensed together with a Mg source under mild conditions without a hydrothermal treatment. It was confirmed that N₂ adsorption–desorption isotherms of the prepared MgO/SBA-15s were classified as type IV, which is characteristic of SBA-15. The X-ray diffraction (XRD) patterns at low diffraction angle also indicated that MgO/SBA-15s retained the ordered mesoporous structure of SBA-15, in spite of a high Mg content. On the other hand, no diffraction peaks due to Mg species were confirmed in the XRD patterns at high diffraction angle, suggesting that Mg species were dispersed on mesopores. After MgO/SBA-15s were impregnated with Co and Mo, the obtained CoMo/MgO/SBA-15 catalysts were tested for ammonia decomposition. It was revealed that the MgO content greatly influenced the catalytic activities of the CoMo-based catalysts. Although a small or large additive amount of MgO decreased the catalytic activity of CoMo/SBA-15, the modification of SBA-15 with 15 wt % of MgO resulted in the highest catalytic performance for ammonia decomposition. Moreover, the present one-pot synthesis method was more favorable for the enhancement of the catalytic activity, compared with the conventional impregnation method.