Series of nanosized ferrite powder Mn1-xZnxFe2O4, 0 ≤ x ≤ 0.9, were prepared through flash combustion using the metal nitrates. The nano-ferrite particles were characterized using different experimental techniques. The defects and microstructure changes of the prepared Mn-Zn ferrite samples were probed using positron annihilation spectroscopy (PAS). The single-phase structure (spinel cubic) for the ferrite samples was confirmed by the x-ray diffraction patterns and Fourier-transform infrared spectra. The cation distribution of Mn1-xZnxFe2O4 was proposed by matching the values of the experimental and theoretical lattice parameters. Also, the B-site cation distribution was modified using the three-sublattice model to adjust the values of the magnetic moments. The results of scanning and transmission electron microscopy (SEM and TEM, respectively) reflect the polycrystalline and nanosized features of the Mn-Zn ferrite nanoparticles, < 26.9 nm. The results reveal that the zinc ions enhance the grain growth from 34 nm to 60 nm. The results showed that the studied samples are soft magnetic ferrites and that their magnetic behavior changes from ferrimagnetic to paramagnetic at higher values of Zn content. The results of the Hall effect measurements revealed the presence of an inverse relationship between charge carrier concentration and zinc content. The results of the PAS measurements revealed that the positrons are trapped and annihilated in di-vacancy-type defects and/or triple junctions present in the ferrite samples. Also, the results suggested that the tetrahedral sites are preferred as the predominant positron trapping sites for high Zn content samples. The correlations between the PAS parameters and other parameters of the experimental techniques used, such as Hall effect measurements, confirmed these findings.