Mixed anion borohydride, closo-borane of potassium, K3BH4B12H12, has been synthesized using mechanochemistry and characterized by combination of temperature dependent synchrotron radiation X-ray powder diffraction, solid state nuclear magnetic resonance, thermal analysis, electrochemical impedance spectroscopy, topology analysis, and ab initio solid state calculations. At RT the compound crystallizes in the monoclinic superstructure (P2/c) of the cubic antiperovskite prototype. At 565 K it transforms by first order phase transition into a rhombohedral (R-3m) deformation of the cubic prototype, which further transforms at 680 K by a second order phase transition into a cubic (P23) antiperovskite structure. While the monoclinic polymorph is observed for the first time among mixed anion salts, the rhombohedral and cubic polymorphs are known among other alkali metal and ammonium halides (or borohydrides), closo-boranes. The first phase transition is related to the repulsive homopolar H−H contacts between BH4 − and B12H12 2− anions which are released at bigger cell volumes, and the orientation of BH4 − anion becomes disordered. The second phase transition is related to orientational disorder of the B12H12 2− anion at bigger cell volumes. The parameters of reorientational motion (activation energies and jump rates) for both BH4 − and B12H12 2− anions in the monoclinic phase were found from the nuclear spin−lattice relaxation measurements. The effect of orientation disorder of both anions on mobility of cations was studied as a case example for the whole family of complex hydrides based on borohydride or closo-borane anions, important solid state electrolytes. While the dynamics of smaller BH4 − anion does not have any measurable effect on K+ mobility, the dynamics and orientation disorder of bigger B12H12 2− is promoting the K+ mobility which would otherwise be limited by the small radius of conducting channels even in the cubic antiperovskite structure.