In order to obtain a deep red emitting Mn4+ doped phosphor for applications in white LEDs, we choose Mg7Ga2GeO12 with AO6 polyhedra favoring the emission of Mn4+ ions as host and design a novel Mg7Ga2GeO12:Mn4+ phosphor by solid state method at 1400 °C in air. The photoluminescence properties were investigated in conjunction with a structural analysis. The crystallographic occupancy of Mn4+ ion in the Mg7Ga2GeO12 host was studied based on the Rietveld refinement results and the crystal chemistry rules as well as luminescence properties. It reveals that the Mn4+ ion occupies the trivalent Ga site in GaO6 polyhedra, resulting in efficient emission of Mn4+ ion. The excitation spectrum exhibits a broad peak at 420 nm due to the characteristic 4A2g→4T2g transitions of Mn4+ ions, and it indicates that this Mg7Ga2GeO12:Mn4+ material can be availably excited in 400–450 nm region. Upon excitation at 420 nm, the composition-optimized Mg7Ga2GeO12:0.5 mol%Mn4+ sample shows broad deep red emission band (2Eg→4A2g transition) in 600–750 nm with the CIE chromaticity (x = 0.7133, y = 0.2866). An analysis of the thermal quenching shows that the emission intensity of the optimal Mg7Ga2GeO12:Mn4+ sample at 250 °C is 78% of their initial intensity at room temperature. Therefore, the Mg7Ga2GeO12:Mn4+ material could serve as a potential red phosphor for white LEDs.