The preparation, structural, microstructural, dielectric, and low temperature magnetic properties of MgxNi1−xFe2O4 (x = 0, 0.17, 0.34, 0.50, 0.66, 0.83, 1) ferrites synthesized by using a self-combustion sol–gel method is presented. Good insulating properties were found for all the compositions at high frequencies (kHz and MHz range), which might drive the present ceramics as interesting for RF/microwaves applications. By increasing the Mg2+ concentration, the total resistivity strongly increases (from ~106 Ωm for the Ni ferrite to 109 Ωm for the Mg ferrite), corresponding to conductivities in the range 10−9–10−6 S/m at f = 1 Hz. Typical ferrimagnetic character with a small coercivity and saturation magnetization in the range (30–50) Am2/kg, which slightly decreases with increasing the Mg content, were found. On the basis of the combined results from the infrared spectroscopy and XRD analysis, it was shown that the magnetic properties depend on the Mg2+ ions distribution on the octahedral and tetrahedral sites and the experimental saturation magnetization allowed to compute the cation distribution for the MgxNi1−xFe2O4 ferrites.