<p>In this study, Mg substituted chromium ferrite nanoparticles of the composition Mg_xCrFe_2−xO₄ (x = 0.00, 0.05, 0.10 and 0.15) were successfully prepared by the sol–gel technique. The influence of Mg substitution on the structural, magnetic, dielectric and photocatalytic properties of chromium ferrite has been investigated systematically. The XRD patterns indicated the formation of a well-defined cubic spinel phase without any impurity peaks, whereas the crystallite size was found to decrease from 30.40&#xa0;nm to 27.40&#xa0;nm with increasing Mg concentration. FT-IR spectra showed two characteristic absorption bands corresponding to tetrahedral and octahedral metal–oxygen vibrations, confirming the ferrite structure. Morphological analysis using FE-SEM and HR-TEM showed the formation of nanosized, nearly spherical particles with slight agglomeration. The BET analysis for x = 0.15 sample showed mesoporous structure with surface area of 10.135 m²/g. The magnetic measurements showed decrease in saturation magnetization from 9.20 to 3.38 emu/g indicating the effect of non-magnetic Mg²⁺ ion substitution on magnetic ordering. Dielectric studies specified normal ferrite behavior with a decrease in dielectric constant and an increase in AC conductivity with frequency. UV–DRS analysis revealed a gradual reduction in band gap energy with Mg substitution, indicating enhanced visible-light absorption and improved charge carrier generation. Photocatalytic activity studied under natural sunlight showed efficient degradation of methylene blue dye with maximum degradation efficiency of 95.64% after 160&#xa0;min for Mg_0.15CrFe_1.85O₄ composition. The improved activity was due to enhanced surface area characteristics and decreased recombination of photogenerated charge carriers. The obtained results demonstrate the suitability of Mg-substituted chromium ferrite nanoparticles as active materials for environmental and electronic applications.</p>