In-plane- and depth-resolved cathodoluminescence (CL) microanalysis and spectroscopy was carried out to study the impact of electron injection on electro-migration and diffusion of native defects and residual impurities in rapidly thermally annealed (RTA) Mg-doped p-type GaN. During intense electron beam irradiation (e.g. Eb, = 10 keV, Ib = 80 nA), an electric field is generated within the primary beam interaction volume. We observed the following two electric field-related effects: (i) an increased electron recombination length and a subsequent field-assisted charge spreading, which causes a dissociation of Mg-H complexes beyond the interaction volume of the primary electron beam, and ii) thermally assisted electro-migration of positively charged, mobile defects leading to the formation of deeper complexes, which are highly stable and act as nonradiative recombination pathways. Furthermore, the diffusion of electron beam-dissociated hydrogen was found to result in the passivation of Mg acceptors beyond the charge recombination volume.