A chiral molecule-based magnet [MnII (S-pnH) (H2O)][MnIII(CN)6]·H2O (S-pn = S-1,2-diaminopropane), 1S·2H2O (P212121), has been obtained, which has a two-dimensional (2D) square network of cyanide-bridged MnII-MnIII ions. The crystallographic research on this coordination polymer indicates that it is robust enough to transform the single-crystal structure upon dehydration as well as rehydration. Meanwhile, the magnetic property changes are reversibly associated with the structural phase transitions. The complete reversibility upon dehydration/rehydration was demonstrated using in situ X-ray diffraction from the as-prepared 1S·2H2O to the dehydrated [MnII (S-pnH)][MnIII(CN)6], 1S, then rehydrated to [MnII (S-pnH) (H2O)][MnIII(CN)6]·H2O, 1S-HP, and dehydrated again to [MnII (S-pnH)][MnIII(CN)6], 1S-DeHP. Neither the space groups nor the crystal axes changed during the dehydration/rehydration process. The dehydration is considered to be associated with a change in the coordination of the S-pnH from one 2D layer to the neighboring one involving a proton transfer and is accompanied by new cyanide bridging of MnII and MnIII ions formed between the 2D sheet. Corresponding to the 2D to three-dimensional (3D) structural transition, the study of magnetic properties reveals that the Curie temperature of long-range magnetic ordering for 1S (TC = 45 K) is more than double that for 1S·2H2O (TC = 21 K).