Dynamic molecular crystals are of high interest due to their potential applications. Herein we report the reversible on-off switching of single-molecule magnet (SMM) behavior in a [Mo(CN)7]4- based molecular compound. Upon dehydration and rehydration, the trinuclear Mn2Mo molecule [Mn(L)(H2O)]2[Mo(CN)7]·2H2O (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear Mn4Mo2 compound [Mn(L)(H2O)]2[Mn(L)]2[Mo(CN)7]2 (2). This structural transformation involves the breaking and reforming of coordination bonds which leads to significant changes in the color and magnetic properties. Compound 1 is an SMM with an energy barrier of 44.9 cm-1, whereas 2 behaves as a simple paramagnet despite its higher ground state spin value. The distortion of the pentagonal bipyramidal geometry of [Mo(CN)7]4- in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.