Structural properties of a quadruple perovskite BiMnO were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above T = 608 K, BiMnO crystallizes in a parent cubic structure with space group Im3̅. Between 460 and 608 K, BiMnO adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as I2/m(o)), and orbital order appears below T. Below T = 460 K, BiMnO is likely to exhibit a transition to space group Im. Finally, below about T = 290 K, a triclinic distortion takes place to space group P1. Structural analyses of BiMnO are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the Im and P1 models is very small, and this fact can explain why the Im to P1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMnO is in striking contrast with that of LaMnO and could be caused by effects of the Bi lone electron pair.