Maintaining high denitration efficiency for the selective catalytic reduction with ammonia (NH3-SCR) at low temperatures is challenging. In this work, a modified biochar-supported perovskite oxide catalyst was synthesized and implemented to NO conversion in the low-temperature range of 100–250 °C. Different modification methods were compared, where the combination of nitric acid and air oxidation treatment endowed biochar with abundant acidic surface oxygen-containing groups and a higher specific surface area as a support. The perovskite oxide (LaMnO3) and the LaMnO3/biochar catalysts were prepared to investigate the interactions between the catalyst and the support. The LaMnO3/biochar catalyst exhibited excellent denitration efficiency and good N2 selectivity, achieving over 80% NO conversion within the entire temperature range of 100–250 °C (S N2 > 90%), and the highest NO conversion reached 95.8% at 225 °C (S N2 = 95.4%). This catalyst provided synergistic adsorption capacity for NH3 as a result of the acidic function of perovskite oxide and acidic oxygen-containing functional groups of the modified biochar support. Additionally, LaMnO3 showed an eminent redox capability for NO conversion due to the high content of Mn4+ and chemically adsorbed oxygen species. Finally, NH3-SCR reaction mechanisms were proposed on the basis of transient response experiments and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) characterization.