Brillouin light scattering and impulsive stimulated light scattering have been used to determine the full elastic constants of magnesiosiderite [(Mg0.35Fe0.65)CO3] up to 70 GPa at room temperature in a diamond-anvil cell. Drastic softening in C11, C33, C12, and C13 elastic moduli associated with the compressive stress component and stiffening in C44 and C14 moduli associated with the shear stress component are observed to occur within the spin transition between ~ 42.4 and ~ 46.5 GPa. Negative values of C12 and C13 are also observed within the spin transition region. The Born criteria constants for the crystal remain positive within the spin transition, indicating that the mixed-spin state remains mechanically stable. Significant auxeticity can be related to the electronic spin transition-induced elastic anomalies based on the analysis of Poisson’s ratio. These elastic anomalies are explained using a thermoelastic model for the rhombohedral system. Finally, we conclude that mixed-spin state ferromagnesite, which is potentially a major deep-carbon carrier, is expected to exhibit abnormal elasticity, including a negative Poisson’s ratio of −0.6 and drastically reduced VP by 10%, in Earth’s midlower mantle.