Biodegradable magnesium alloys generally contain intermetallic phases on the micro‐ or nanoscale, which can initiate and control local corrosion processes via microgalvanic coupling. However, the experimental difficulties in characterizing active degradation on the nanoscale have so far limited the understanding of how these materials degrade in complex physiological environments. Here a quasi‐in situ experiment based on transmission electron microscopy (TEM) is designed, which enables the initial corrosion attack at nanometric particles to be accessed within the first seconds of immersion. Combined with high‐resolution ex situ cross‐sectional TEM analysis of a well‐developed corrosion‐product layer, mechanistic insights into Mg‐alloys' degradation on the nanoscale are provided over a large range of immersion times. Applying this methodology to lean Mg–Zn‒Ca alloys and following in detail the dissolution of their nanometric Zn‐ and Ca‐rich particles the in statu nascendi observation of intermetallic‐particle dealloying is documented for magnesium alloys, where electrochemically active Ca and Mg preferentially dissolve and electropositive Zn enriches, inducing the particles' gradual ennoblement. Based on electrochemical theory, here, the concept of cathodic‐polarization‐induced dealloying, which controls the dynamic microstructural changes, is presented. The general prerequisites for this new dealloying mechanism to occur in multicomponent alloys and its distinction to other dealloying modes are also discussed. Dealloying of intermetallic nanoprecipitates governs the electrochemical reactivity of multicomponent Mg alloys. TEM‐based analyses allow the direct observation of intermetallic‐particle (IMP) dealloying at the nanoscale. Electrochemically active Ca preferentially dissolves, while electrochemically more noble Zn is cathodically protected. Zn enrichment leads to a gradual ennoblement of the IMP concomitant with its enhancing cathodic reactivity.