Transition metal nitrides have great potential applications as incompressible and high energy density materials. Various polymeric nitrogen structures significantly affect their properties, contributing to their complex bonding modes and coordination conditions. Herein, we first report a new manganese polynitride MnN4 with bifacial trans-cis [N4]n chains by treating with high-pressure and high-temperature conditions in a diamond anvil cell. Our experiments reveal that MnN4 has a P-1 symmetry and could stabilize in the pressure range of 56-127 GPa. Detailed pressure-volume data and calculations of this phase indicate that MnN4 is a potential hard (255 GPa) and high energy density (2.97 kJ/g) material. The asymmetric interactions impel N1 and N4 atoms to hybridize to sp2-3, which causes distortions of [N4]n chains. This work discovers a new polynitride material, fills the gap for the study of manganese polynitride under high pressure, and offers some new insights into the formation of polymeric nitrogen structures.