In this work, we prepared a reduced graphene oxide (rGO)-supported manganese ferrite (MnFe2O4) hybrid material by a simple one-pot solvothermal synthesis method, using graphite oxide (GO) and metal ions (Fe3+, Mn2+) as raw materials. The reduction of GO and the in situ formation of MnFe2O4 nanoparticles were simultaneously achieved during the synthesis. The properties of MnFe2O4/rGO were characterized by scanning electron microscopy, powder X-ray diffraction, Fourier-transform infrared spectroscopy, and energy-dispersive X-ray spectrometry. The electrochemical characterizations of the resulting sensor were carried out by cyclic voltammetry and chronoamperometry. The results of electrochemical experiments show that the composite has improved hydrogen peroxide (H2O2) reduction performance. The linear range of the as-prepared sensor for H2O2 detection is 0.025 to 1.5 mM, with a detection limit of 0.796 μM (S/N = 3) and a response time of less than 4 s. In this paper, an effective, economical, and green experimental method for the synthesis of metal-oxide/graphene nanocomposites is proposed.