We report the preparation, characterization, and magnetic hyperthermia of magnesium-doped maghemite (γ-Fe2O3) nanoparticles (NPs). XRD and Rietveld refinement reveal the formation of a single cubic phase of γ-Fe2O3, where the substitution of Fe+3 located in the tetrahedral site by Mg+2 did not alter the crystal structure. Magnetic measurements showed an increase of saturation for low concentration of magnesium (1% and 3%) and a decrease for 5% of magnesium. The superparamagnetism behavior and the effective anisotropy constant (\({K_{eff}}\)) were confirmed and determined by the Langevin model and the law of approach to saturation (LAS), respectively. Hyperthermia under an alternating magnetic field (AMF) was conducted, indicating that all the NPs reach hyperthermia temperatures (42 °C) in relatively short times (3–5 min.). It was found that all the Mg-doped γ-Fe2O3 NPs present relatively good heating efficiencies, with SAR values in the range of 35–114W/g and ILP values (0.58–1.67 nHm2/kg). Furthermore, the linear response theory (LRT) model was studied to further assess heating mechanisms and to determine the Néel relaxation time (\({\tau }_{R}\)). Our finding strongly suggests that the as-prepared Mgx-doped γ-Fe2O3 MNPs are promising for hyperthermia application.