Magnetic anisotropy and spin reorientation transition (SRT) are fundamental physical factors governing magnetic performance and tunability in spintronic applications. By manipulating magnetic anisotropy, one can achieve the controlled switching of magnetization orientation, tuning of coercivity, and optimization of magnetic response behavior. However, in layered MnBi, the competition between Mn 3d-Bi 6p hybridization and the strong spin–orbit coupling (SOC) related to Bi leads to a competition for anisotropy contributions, which causes widespread debate on the microscopic origin of its SRT. In this work, we successfully synthesize millimeter-sized high-quality MnBi single crystals by using a flux method and systematically investigate the evolution of magnetic anisotropy through temperature-dependent magnetometry, transport measurements, variable-temperature X-ray diffraction, and high-pressure experiments. The results reveal that MnBi undergoes a spin reorientation from the in-plane direction to the c-axis at approximately 90 K. At the same time, an abrupt anomaly in the lattice c/a ratio emerges near 140 K prior to the magnetic transition, which indicates the presence of a structural precursor state and a strong coupling between lattice and magnetic degree of freedom. Transport measurements further demonstrate that when the magnetic field is applied parallel to the ab-plane, the magnetoresistance (MR) switches from positive to negative near the SRT, confirming that magnetic reconstruction directly modulates carrier scattering mechanism. Under external pressures ranging from 0.9to 3 GPa, the SRT temperature progressively decreases whereas the magnetization along the c-axis is markedly enhanced. This behavior indicates that pressure modifies the competition between Mn-Bi electronic hybridization and Mn–Mn direct exchange interaction by tuning lattice contraction, ultimately stabilizing the c-axis magnetic orientation. Overall, this study clarifies the coupling effect of structural evolution, electronic hybridization, and magnetic anisotropy in MnBi, and demonstrates that external-field engineering provides an effective route for modulating anisotropy. These findings provide valuable insights for designing high-anisotropy permanent magnets and tunable spintronic materials