Next-generation bioelectronic implants require miniaturization, durability, and long-term functionality. Thin film encapsulations, prepared with inorganic or hybrid organic/inorganic designs, are essential for ensuring protection, low water permeation, adaptability, and structural strength. It is equally important to precisely measure their barrier performance, especially for in vivo use, to ensure the manufacture of reliable bioelectronics. Current monitoring solutions are not adequate: they are bulky, lack sensitivity, and are incompatible with microfabricated devices. Here, a comprehensive method is introduced to quantify the permeability of thin-film encapsulation for bioelectronic implants both in situ and in real time. This method relies on monitoring the electrical resistance of the Mg film, which experiences corrosion due to water permeation, leading to Mg hydrolysis. An analytical model is proposed that predicts and quantifies this permeation, and is adaptable for various types of encapsulations, including hybrid multilayers. An unprecedented ultra-low detection limit of 3 x 10-8 g m-2 d-1 at room temperature is demonstrated and the monitoring approach is validated in vivo using polyimide and poly(dimethylsiloxane)-coated bioelectronics.