We have reported the effect of Cr³⁺ ion doping on the structural, ac transport and Mössbauer spectral studies of ferrite nanoparticles Ni_0.5Mg_0.5Fe_2-xCr_xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) prepared by sol–gel auto combustion technique. The micro-structural studies of the samples were obtained by using scanning electron microscopy (SEM). The dependence of Mössbauer parameters, viz., line width, isomer shift, quadrupole splitting and hyperfine magnetic field on Cr³⁺ substitution have been analysed. Hyperfine magnetic field and isomer shift values decreases whereas line width increases with Cr³⁺ substitution. Rietveld refinement and Mössbauer spectroscopy studies reveal that the Ni_0.5Mg_0.5Fe_2-xCr_xO₄ with various concentrations of x belongs to mixed spinel structure. Nèel’s molecular field model is used to explain the variation of hyperfine magnetic field with increasing Cr³⁺ substitution.