Narrow band green emitting phosphors have gained widespread attention due to their application in white light emitting diode (wLED) backlight displays. Commercial backlight displays have a broad band green phosphor which limits their performance. In this work, bright, narrow and thermally stable green emitting MgGa2O4:Mn2+ (MGO-Mn) has been synthesized. Time-resolved emission spectroscopy suggested that Mn2+ ions are distributed at both Mg2+ and Ga3+ sites of the MGO spinel, which resulted in a high internal quantum efficiency of 63%. The colour purity of MGO-Mn (76.4%) superseded that of the commercial green phosphor β-SiAlON:Eu2+ (59.12%). Doping-induced creation of oxygen vacancies endows MGO-Mn with excellent persistent luminescence with a time duration of more than 900 s upon 4 min charging with 270 nm UV light and persistent radioluminescence of more than 6000 s when charged with X-rays for 1 min. Finally, tunable white LEDs (cool and neutral white LEDs) are fabricated by combining the RGB mixture of the green phosphor with commercial red and blue phosphors along with a 280 nm UV LED chip. This work also showcases the importance of different annealing atmospheres in the photoluminescence and persistent luminescence of the MGO-Mn phosphor.