Using in situ scanning transmission electron microscopy (STEM) and low-loss plasmon electron energy-loss spectroscopy (EELS), we reveal asymmetric transformation mechanisms during the hydrogenation and dehydrogenation of Mg thin films. Remarkably, during hydrogenation, the MgH2 phase can nucleate from either the bottom or top interface of a Mg thin film while symmetrically sandwiched between two Ti layers. This unexpected behavior, occurring under identical external conditions, highlights the critical role of nucleation barriers in the phase transformation process, challenging conventional diffusion-driven paradigms. In contrast, dehydrogenation proceeds exclusively via an H2 diffusion-controlled frontal growth originating from the top interface. These insights underscore the importance of understanding metal-to-metal hydride phase transformations for advancing hydrogen storage technologies and applications such as hydrogen sensing.