We have studied in this work the effect of increasing structural disorder on the persistent luminescence of a Cr³⁺ doped zinc gallate spinel. This disorder was introduced by progressive substitution of Zn²⁺ by Mg²⁺ ions, and was studied by photoluminescence, X-ray diffraction, extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) spectroscopy. It was found that increasing the Mg/Zn substitution decreases the number of Cr³⁺ in undistorted sites and increases the number of Cr³⁺ with neighbouring antisite defects and with neighbouring Cr³⁺ ions (referred to as Cr clusters), which in turn decreases the intensity of persistent luminescence. Both XANES and EPR spectra could be simulated by a linear combination of Cr³⁺ spectra with three types of Cr³⁺ environments. The increasing disorder was found to be correlated with a decrease of the average Cr–O bond length and a decrease of crystal field strength experienced by Cr³⁺ ions.