A series of nickel and zirconium co-substituted zinc-cobalt spinel ferrites with the chemical formula Zn0.25Co0.75(NiZr)xFe2-2xO4 (0.05 ≤ x ≤ 0.25, Δx = 0.05) was successfully synthesized using the sol-gel citrate route to curb the rapidly growing electromagnetic pollution caused due to 5G technologies. A comprehensive study of the structural, morphological, magnetic, optical, electromagnetic, and microwave absorption characteristics was carried out to analyze the effect of Ni2+ and Zr4+ ionic substitution. X-ray diffraction confirmed the formation of nanoparticles having a cubic spinel structure with an Fd-3m space group. Magnetic parameters like saturation magnetization and remanent magnetization were found to decrease with increasing the level of substitution. The permittivity, permeability, and absorption properties were investigated in the Ka-band (26.5–40 GHz). Composition x = 0.10 yielded an optimum value of reflection loss up to −50.34 dB at a matching frequency of 29.20 GHz and a matching thickness of 1.3 mm. The results obtained suggest that the synthesized materials prove to be good microwave absorbers for next-generation devices and applications.