Scanning cathodoluminescence (CL) spectroscopy and imaging were used to study the effect of post-growth processing on the CL efficiency of metal-organic vapour phase epitaxy-grown Mg-doped GaN. In this work, two treatments, thermal annealing in high-purity gaseous atmospheres (N2, O2 and H2(5%)/N2) and low-energy electron beam irradiation (LEEBI), have been investigated. Post-growth annealing in a H2/N2 atmosphere followed by LEEBI leads to a significant enhancement of the free electron-to-bound Mg-acceptor (e, Mg) CL emission and a reduction of nonradiative centres involving native defects. The presented results demonstrate that the combination of post-growth annealing in a H2/N2 atmosphere and LEEBI dissociation of Mg-H complex acceptors significantly improves the light emitting efficiency of Mg-doped p-type GaN. Conversely, the samples annealed in a N2 or O2 atmosphere exhibit a reduced (e, Mg) emission after both annealing and LEEBI treatment.