Long-life MnO-based anodes with high power density for lithium ion batteries (LIBs) is still a great challenge due to the inferior electrical conductivity and drastic volume change of MnO during the lithiation/delithiation process. Herein, to achieve high rate capacity and long cycle life simultaneously, MnO nanoparticles were encapsulated within a foam-like carbon nanosheet matrix through a confined phase transition process. By annealing polydopamine (PDA) coated porous ZnMnO3 nanosheets in a reducing atmosphere, MnO nanoparticles and surrounding in-situ formed pores could be encapsulated in the PDA-derived carbon nanosheets synchronously, forming MnO@C nanosheets with ball-in-pore structure (MnO@C-BP). The internal voids originating from Zn evaporation can not only accommodate the huge volume expansion of MnO nanoparticles, but also significantly enlarge the specific surface area, leading to an enhanced pseudocapacitive Li-storage behavior. Moreover, the as-prepared sheet-shaped MnO@C-BP with thickness of about 30 nm can provide a short path for fast Li-ion diffusion, while the continuous carbon framework promotes the charge transfer between the encapsulated MnO nanoparticles. Because of these merits, MnO@C-BP electrode exhibits superior rate performance (514 mAh g−1 at 10 A g−1, 383 mAh g−1 at 15 A g−1), outstanding cycling performance (1212 mAh g−1 after 1000 cycles at 2 A g−1, capacity retention of 127%), as well as a high reversible capacity of 1178 mAh g−1 at 0.1 A g−1.