Flexible luminescent crystals endowed with mechanical compliance are emerging as materials that could be the foundation of future lightweight single-crystal flexible optoelectronics. Multiple mechanical responses (for example, elastic and plastic deformation) are rarely observed with the same material among the reported examples of such materials. Here, we report a Schiff base, (Z)-3-(4-ethoxyphenyl)-2-(4-(((E)-2-hydroxy-5-methoxybenzylidene)amino)phenyl)acrylonitrile, which crystallizes as two polymorphs and one tetrahydrofuran solvate. All three forms are emissive, but they have different mechanical properties. Specifically, two of the forms that are unsolvated polymorphs (denoted A and B) were found to be brittle and plastic, respectively, while the third form, which is a solvate (denoted C), showed excellent elasticity. Notably, form C becomes plastic after the crystal is desolvated. Single-crystal X-ray diffraction (SCXRD) and mechanical testing were performed to obtain better insight into the root-cause for the observed difference in mechanical properties. Since crystals of forms B and C are mechanically compliant as well as optically transparent, they were tested as flexible single-crystal optical waveguides.