New garnet phosphors, Lu3−xYxMgAl3SiO12:Ce3+ (x = 0–3), which can be efficiently excited by blue light and emit the yellow-orange light, were developed using the solid solution design strategy combining the chemical unit substitution and the cation substitution. Crystal structures of the four compounds were reported for the first time via the Rietveld refinement of their powder XRD patterns. All phosphors show the general cubic garnet structure with the space group Iad. The specific occupancy of Lu/Y, Al/Mg, Al/Si and O atoms in different positions was identified. The evolution of cell parameters and Y/Lu/Ce–O bond lengths were identified. Photoluminescence properties were evaluated on aspects of emission/excitation spectra, internal/external quantum efficiency and thermal emission stability. Under the 450 nm blue light excitation, the phosphors exhibit bright yellow color emission, peaking in the 575–597 nm spectral range. The internal and external quantum efficiency can reach 83% and 58%, respectively. The emission red-shift in response to the Y/Lu ratio variation was discussed in relation to the local structure evolution. The phosphors are relatively promising to act as wavelength converter of blue light in white light emitting diodes.