The synthesis of a highly ordered mesoporous ternary Mg–Nb oxide by the Evaporation Induced Self-Assembly (EISA) method is presented for the first time. The as-prepared material shows 2D-hexagonal pore periodicity, with unimodal pore size distribution centered at 5.5 nm, and a large surface area (169 m2 g−1). The mechanical properties, determined by means of nanoindentation, are consistent with the presence of ordered domains of honeycomb-like hexagonal pore arrangements, in agreement with electron microscopy observations and N2 sorption isotherm analyses. The dielectric constant of the mesoporous sample, measured at room temperature in the frequency range 1 kHz to 1 MHz, is rather high (κ 25 at 1 MHz) and correlates well with the κ value of the bulk and the porosity level of this material. Moreover, the thermal stability of the mesoporous magnesium niobate is investigated after air-annealing treatments at different temperatures. While a significant decrease of the surface area is observed for TANN = 650 °C, the mesostructure fully collapses after annealing at 800 °C.