Two novel polymeric Mg formates [(Fmd)Mg(HCOO)₃]_∞ (1) and [(Gua)Mg(HCOO)₃]_∞ (2) containing the formamidinium [Fmd⁺, (NH₂–CH⁺–NH₂)] and guanidinium [Gua⁺, C⁺(NH₂)₃] cations have been prepared under solvothermal conditions. 1 and 2 are isostructural; they crystallize in the orthorhombic space group Pnna. Their 3D scaffolds consists of Mg ions in an octahedral coordination environment bridged by formate ligands. The overall framework charge is negative, and the cations are located in the centre of the lattice cavities, forming extensive N–H⋯O hydrogen bonding with the surrounding cage. Both compounds have been characterized through single-crystal and powder X-ray diffraction, 1D and 2D solid-state NMR (¹H, ¹³C and ¹⁵N), IR spectroscopy and TG-MS analysis. Their chemical reactivity towards ion exchange and ability as CO₂ storage materials has been finally examined.