Nitrile butadiene rubber (NBR) samples filled with cobalt–zinc (Co-Zn) ferrite nanoparticles (Co_1−xZn_xFe₂O₄, where x = 0, 0.2, 0.4, 0.6, 0.8, 0.9, 0.95 and 1) were prepared. The structure and morphology of Co-Zn ferrite nanoparticles were investigated using X-ray diffraction, transmission electron microscopy, and Fourier transform infrared (FTIR) spectroscopy, while the structure of the Co-Zn nanoferrite-filled NBR composites was studied using scanning electron microscopy and FTIR spectroscopy. The influence of ferrite composition on cure characteristics, mechanical properties and hardness showed an improvement up to x = 0.8. Dielectric parameters showed an enhancement with ferrite composition.