We investigated the impact of cobalt (Co) incorporation on the structural, optical, and magnetic properties of ferrite Ni1 − xCoxMn0.05Fe1.95O4, also known as NCMF for x = 0, 0.02, and 0.04, which were successfully synthesized using a low-temperature sol gel combustion process with nitrates as cations and citric acid (C6H8O7) as the combustion/chelating agent. In X-ray diffraction studies, cobalt incorporation resulted in a significant increase in lattice parameter from 8.31 to 8.36, as well as a significant increase in density from 5.42 to 5.52 g/cm3; crystal size formation was observed in the range of 44.76 nm to 48.54 nm for all samples. Raman spectra analysis confirmed that single phase belongs to the Fd3m space group. Fourier transform infrared (FTIR) spectra were used to identify functional clusters and residual groups in all samples. UV-visible spectroscopy showed a redshift of 0.92 eV for x = 0.04. The effect of dopants on nickel ferrites' magnetization was observed in the range 52emu/g − 59emu/g, indicating that all samples contain soft magnetic content. Antifungal activity of synthesized sample was analyzed against Aspergillus niger (MT675916) species of plant pathogenic fungi isolated from Capsicum during storage. Biological analysis designated that the sample for x = 0.04 has shown a 100% potent antifungal activity against Aspergillus niger (MT675916) species. For x = 0.04 treatment completely inhibit the growth of fungus after 7 days. Therefore, NCMF NPs can be used as applicant resources for industrial, medical, and biological applications.