The influence of Mg2+-Si4+/Ge4+ incorporation into Ce3+-doped Y3Al5O12 garnet phosphors on the crystal structure and luminescence properties is described in this work. X-ray diffraction with Rietveld refinements, photoluminescence spectra, absolute quantum yield, thermal quenching behavior, and lifetimes were utilized to characterize samples. The introduction of Mg2+-Si4+/Ge4+ leads to an obvious red shift of emission wavelength under the excitation of blue light, especially for the series of Mg2+-Si4+ substitutions, which is suited for white light-emitting diodes (LEDs) with low color temperatures and good color rendering using only a single phosphor. More interestingly, an additional emission band locating at high-energy was observed with ultraviolet excitation, which is different than previous literature. Under the excitation of ultraviolet, the emission color for the Mg2+-Si4+ substitutions can be tuned from yellow-green to blue, which is expected to obtain single-phased phosphors with white emission excited with UV-LED chip. The usual Ce3+ emission band at low energy has stronger quenching at high temperatures. The mechanisms for the observed phenomena are discussed.