Auxiliary activity family 9 (AA9) lytic polysaccharide monooxygenases (LPMOs) show significant synergism with cellulase in cellulose degradation. In recent years, there have been many reports on AA9 LPMOs; however, the identification of efficient and thermostable AA9 LPMOs with broad potential for industrial applications remains necessary. In this study, a new AA9 LPMO from Talaromyces cellulolyticus, named TcAA9A, was identified. An analysis of the oxidation products of phosphoric acid-swollen cellulose categorized TcAA9A as a type 3 AA9 LPMO, and TcAA9A exhibited a better synergistic effect with cellulase from Trichoderma reesei than what is seen with TaAA9A, a well-studied AA9 LPMO from Thermoascus aurantiacus. Two AA9 LPMOs were successfully expressed in T. reesei, and the transformants were named TrTcAA9A and TrTaAA9A. The activities and thermostabilities of the AA9 LPMOs in TrTcAA9A were higher than those of the AA9 LPMOs in TrTaAA9A or the parent. The enzyme solution of TrTcAA9A was more efficient than that of the parent or TrTaAA9A for the degradation of Avicel and delignified corncob residue. Thus, TcAA9A showed a better performance than TaAA9A in T. reesei cellulase cocktails. This study may offer an innovative solution for improving enzyme cocktail activity for lignocellulosic degradation.