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Highly active samarium manganese perovskite oxides were successfully prepared by employing self-molten-polymerization, coprecipitation, sol–gel, and impregnation methods. The physicochemical properties of perovskite oxides were investigated by XRD, N2 adsorption–desorption, XPS, and H2-TPR. Their catalytic performances were compared via the catalytic oxidation of toluene. The perovskite prepared by self-molten-polymerization possessed the highest catalytic capacity, which can be ascribed to its higher oxygen adspecies concentration (Olatt/Oads = 0.53), higher surface Mn4+/Mn3+ ratio (Mn4+/Mn3+ = 0.95), and best low-temperature reducibility (H2 consumption = 0.27; below 350 °C). The most active catalyst also exhibited good cycling and long-term stability for toluene oxidation. After a multistep cycle reaction and a long-term reaction of 42 h, the toluene conversion maintained above 99.9% at 270 °C. Mechanistic study hinted that lattice oxygen was involved in toluene oxidation. The oxidation reaction was dependent on the synergism of lattice oxygen, adsorbed oxygen, and oxygen vacancies. The degradation pathway of toluene, researched by diffuse reflectance infrared Fourier transform spectroscopy and online mass spectrometry technologies, demonstrated that a series of organic byproducts existed at a relatively low temperature. This work provides an efficient and practical method for selecting highly active catalysts and for exploring the catalytic mechanism for the removal of atmospheric environmental pollution.