In this study, magnetic polymer nanocomposite comprising of polylactide, liquified natural rubber and different loadings of NiZn ferrite nanoparticles was prepared using ultrasonication-assisted melt compounding method. The fabrication process has involved the ultrasonication treatment of the nanocomposite for 1 and 2 h. The influence of ultrasonication on the dispersion degree of the nanoparticles and its reflection on the behaviour of PLA/LNR/NiZn nanocomposites was evaluated. The tensile properties showed an improvement when the nanocomposites were treated for 1 and 2 h, with optimum values obtained at 4 wt% of nanofiller loading. Similarly, the optimum values of Tg, Tc and Tm obtained from DSC analysis were found for sample ultrasonically treated for 1 h with highest crystallinity increment of 55.16% as compared to untreated sample of the same nanofiller loading. Moreover, the statistical analysis based on TGA and SEM results revealed that the nanocomposite treated ultrasonically for 1 h had better nanofiller dispersion as compared to its counterparts with 0 and 2 h treatment periods. These findings, as confirmed by magnetic testing, formed a solid evidence that the optimal behaviour of PLA/LNR/NiZn nanocomposites was attained with ultrasonication treatment of 1 h with potential applications in areas served by soft magnetic materials.