Mn4+-Doped oxide phosphors are under intensive investigation owing to their low manufacture cost and attractive luminescent features for indoor plant cultivation applications. However, it is still a challenge to develop Mn4+-doped oxides with high luminescence efficiency and thermal stability. Herein, Mn4+-Mg2+ pairs are incorporated into a CaGdAlO4 host to reduce non-radiative channels formed by Mn4+-Mn4+-O2- clusters. The photoluminescence and quantum efficiency are significantly enhanced after the introduction of Mg2+ ions to the host. A prolonged Mn4+ decay time is also obtained from the Mn4+/Mg2+ co-doped samples. Intense red emission with a narrow peak at 712 nm due to the 2Eg → 4A2g transition of Mn4+ ions is observed under 335 nm excitation. LEDs fabricated by coating the synthesized phosphor on a 365 nm near-UV chip exhibit an intense deep-red emission with CIE chromaticity coordinates of (0.712, 0.285). The results indicate Mn4+/Mg2+ co-doped CaGdAlO4 phosphors may be applicable to plant cultivation fields.