We report a new Mg2+, Mn4+ co-doped red-emitting SrLaMgTaO6 phosphor for solid state lighting. The phosphor can be efficiently excited in the wavelength range of 250-500 nm, which matches well with the emission wavelengths of blue or NUV chips. The photo-luminescence spectra exhibit the highest emission peak at 692 nm from E-2(g) -> (4)A(2) spin forbidden transition of Mn4+ under NUV excitation. The Mg2+ co-dopant for charge compensation plays a key role in enhancing the luminescence intensity by a factor of 2.8. The incorporation of Mg2+ ion in the host could not only suppress the energy migration among Mn4+ ions but also eliminates the charge imbalance in Mn4+ - Mg2+ pairs. W-LED fabrication with different mass ratios of yellow emitting YAG: Ce3+ and red emitting SrLaMgTaO6:Mn4+/Mg2+ phosphors is designed to explore its possible application in warm w-LEDs with high CRI. The w-LEDs fabricated with the integration of SrLaMgTaO6:Mn4+/Mg2+ red phosphor show changing trends toward warm white light with lower color temperature and higher CRI than that based on single yellow emitting YAG:Ce3+ phosphors. The results suggest that SrLaMgTaO6: Mn4+/Mg2+ phosphor has great potential for the warm w-LEDs application as a suitable red light component.