<p>Nickel–Zinc (Ni-Zn) ferrites are promising magnetic materials for technological applications; however, further enhancement of their magnetic performance through cation substitution remains an important research challenge. In this study, cobalt-substituted Ni–Zn ferrites were synthesized using the sol–gel auto-combustion method to investigate the influence of Co²⁺ incorporation on their structural, microstructural, and magnetic properties. Thermogravimetric–differential thermal analysis (TG–DTA) was employed to determine the appropriate calcination temperature, while X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and magnetic measurements were used for comprehensive characterization. Rietveld-refined XRD patterns confirmed the formation of a single-phase cubic spinel structure (<i>Fd–3&#xa0;m</i>) with high phase purity. A slight decrease in lattice parameter and porosity, accompanied by an increase in grain and particle sizes, was observed with increasing Co²⁺ content. Cation distribution analysis indicated the preferential occupation of octahedral (B) sites by Co²⁺ ions, leading to redistribution of Fe³⁺ ions and modifications in metal–oxygen bonding, as supported by FTIR results. Magnetic measurements revealed a significant enhancement in magnetic performance, with saturation magnetization increasing from 50.56 to 61.76 emu g⁻¹. The increases in remanent magnetization, coercivity, remanence ratio, and Bohr magneton number were attributed to enhanced ferrimagnetic ordering, cation redistribution, reduced spin disorder, and increased magnetic anisotropy. These findings demonstrate that Co²⁺ substitution effectively improves the structural and magnetic characteristics of Ni–Zn ferrites, making them attractive candidates for advanced magnetic applications.</p>