Manufacturing of novel transformer oil-based nanofluids has become necessary to improve dielectric and cooling properties. The current study was carried out to examine the dielectric and thermal characteristics of transformer oil incorporated by Mn_0.5 Zn_0.5 Fe₂O₄ nanoparticles (Mn-Zn ferrite NPs) that were synthesized by an auto-combustion technique. The microstructure and morphology of the synthesized Mn-Zn ferrite NPs were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), and high-resolution transmission electron microscopy (HRTEM). The formation of a single-phase cubic spinel structure for the Mn-Zn ferrite NPs was confirmed by XRD results. The average crystallite sizes of about 26.7 nm obtained from XRD are very close to the measured value obtained from HRTEM. VSM measurements ensure a high saturation magnetization of 57 emu/g for the Mn-Zn ferrite sample and which makes it a very good candidate as a transformer oil filler. The as-synthesized Mn-Zn ferrite NPs were then introduced into transformer oil with concentrations up to 0.4 g/L to form Mn-Zn nano ferrite-fluid. The dielectric properties of the prepared nano ferrite-fluid samples were investigated by measuring the dielectric constant (ε′) and dissipation factor (tan δ) in the frequency range of 30 Hz − 1 MHz. Moreover, the AC breakdown voltage of the investigated nano ferrite-fluid samples was measured at a constant 500 V/s ramp, and it showed a pronounced enhancement with the addition of Mn-Zn ferrite NPs. It reached 80 % for 0.2 g/L of NPs. Moreover, the convective heat transfer of the nano ferrite-fluid was examined through the heating–cooling process, which is indicating a significant reduction in the thermal time constant in cooling with the introduction of the Mn-Zn ferrite NPs into the oil.