The initial intensity and afterglow duration of existing long-wavelength persistent phosphors are insufficient for practical applications. Herein, the afterglow of orange-emitting Sr 3 Al 2 O 5 Cl 2 :Eu 2+ , Dy 3+ phosphors are enhanced by co-doping with Mg 2+ using the conventional solid-state reaction. Rietveld refinement for the prepared phosphor revealed that all Eu 2+ cations were placed on the Sr 2+ sites, whereas Dy 3+ occupancy in the lattice was improved by incorporating Mg 2+ . All phosphor samples with different Mg 2+ concentrations emit 609-nm-centred orange luminescence caused by the 5d→4f transition of Eu 2+ . The photoluminescence was significantly enhanced with the increase in the Mg 2+ doping due to the increased amount of divalent Eu 2+ in the samples, as confirmed by the X-ray photoelectron spectroscopy investigation. Incorporating Mg 2+ also enhanced the afterglow performance by creating suitable electron traps in the phosphor. The afterglow profiles were simulated by a double exponential decay model, and the results revealed that significant improvement in the afterglow lifetime was achieved for the 0.3 Mg 2+ -doped phosphors. Also, Mg 2+ introduction resulted in a deeper trap level (∼0.72 eV) and an increased trap concentration. Consequently, the Mg 2+ -doped exhibits brighter initial photoluminescence and longer afterglow. • Sr 3−x Mg x O 5 Cl 2 :Eu 2+ , Dy 3+ phosphor exhibits orange emission at 609 nm. • The photoluminescence is improved due to the increased Eu 2+ content after Mg 2+ doping. • Enhanced afterglow is observed after the incorporation of Mg 2+ cations. • The improved afterglow is caused by the increased trap concentration and modified trap depth.