Evaluation of damage evolution effects in biodegradable pure Mg was carried out, using transmission electron microscope as surrogate irradiation for high-energy radionuclide β decay in Brachytherapy. Time-dependent quantitative defect production, evolution dynamics, and evolution statistics were revealed in-situ for two prism foils (z = [1210], [1010]), in as-received and heat-treated pure Mg, after 300 keV electron irradiation up to 0.468 dpa at R.T. Preferred nucleation of basal-plane interstitial-type 1/6 <2023> loops was confirmed, in addition to a small portion of prism-plane 1/3 <1120> loops. No cavities were found. A higher yield of point defect concentration and a more evident trend of defect coarsening were identified in [1210] than in [1010]. Pre-existing dislocations (on the orders of 10¹³−10¹⁴ m⁻²) in pure Mg resulted in a delay of the first occurrence of visible defects. Defect migration and elastic interactions governed the microstructural evolution of electron irradiation damage in pure Mg, giving rise to events of loop coalescence, growth, and sometimes rotation of habit plane. The influence of incident electron energy can be correlated to the rates of point defect production, and is quantifiable; however, interfered by defect cluster stability, defect mobility, and defect interactions. This forms an important theoretical basis for the application of Mg subjected to MeV-level β-decay radiation in Brachytherapy. The paper concludes with a brief comparison between Mg and conventional Ti casing, outlines the advantages and challenges, and provides reference points for the validation of Mg/Mg-alloys in Brachytherapy seed application.