Need for optical limiting devices that work on the basis of third order optical non-linearity has necessitated the search for different types of materials. Ferrites which show excellent magnetic properties are found to show optical non-linearity and have been exploited for this optical non-linearity in recent times. Current work explores the non-linear optical properties of nickel substituted manganese ferrites synthesized by wet chemical co-precipitation method. Structural studies on these samples indicated the formation of pure spinel ferrites with reduction in their crystallite sizes with increasing nickel concentration. The variation in particle size is similar to the measured optical band gap which is corroborated with the quantum size effect. Information on the presence and effects of defects in these samples are obtained from photoluminescence measurements. Z-scan measurements yielded non-linear optical parameters with their third order nonlinear susceptibility of the order of 10−6 esu. The observed self-defocusing effect was utilized to describe the optical limiting property with optical limiting threshold to be less than 2.5 mW for nickel substituted manganese ferrite nanoparticles under 532 nm CW laser. © 2018 Elsevier Ltd and Techna Group S.r.l.