[en] The cathodoluminescence (CL) characterizations of Lu${}_3$Al${}_5$O${}_{12}$:Ce,Mg (LuAG:Ce,Mg) and of multicomponent (Lu${}_{0.25}$Gd${}_{0.74}$)${}_3$(Ga${}_{2.48}$Al${}_{2.52}$)O${}_{12}$:Ce,Mg (LuGAGG:Ce,Mg) single‐crystalline garnet films are supported by the optical absorption spectroscopy. Using this approach, the influence of the Mg‐to‐Ce concentration ratio on the CL spectra and the CL decay is analyzed. The crucial role of stable Ce${}^{4+}$ centers in CL is shown in Mg‐rich film studies. Although the CL intensity is somewhat reduced, drastic improvements in timing performance are demonstrated, including the CL decay time as low as 4.2 ns and the afterglow as low as 0.015% at 500 ns after the e‐beam excitation cutoff. These results predetermine the utilization of the LuAG:Ce,Mg and LuGAGG:Ce,Mg single‐crystalline films in e‐beam detection systems, where especially fast scintillator response is crucial. (© 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)