A novel Sm3+-doped ZnLaB5O10 phosphor was produced using a high-temperature solid-phase synthesis method. The Rietveld refinement of the ZnLaB5O10:Sm3+ phosphor was carried out and its successful synthesis was demonstrated. The Sm3+ ion had a 6H5/2-4F7/2 transition at 402 nm. The emission intensity of the prepared phosphor at 596 nm was greater than that at 562 nm, which was due to occupying an asymmetric position. The optimized doping concentration of the ZnLaB5O10:xSm3+ sample was determined to be 5 mol%, and the relevant mechanism of concentration quenching was demonstrated as the dipole-quadrupole interaction. The internal quantum efficiency of ZnLaB5O10:5 mol%Sm3+ was 61.19%. In the experimental range of 300–480 K, ZnLaB5O10:Sm3+ phosphor demonstrated high thermal stability, and had almost no thermal quenching phenomenon. Then, the fabricated warm white light-emitting diodes (w-LEDs) with ZnLaB5O10:Sm3+ phosphors presented an excellent average colour rendering index (Ra = 92, CCT = 5332 K). These results indicated the promising usage of ZnLaB5O10:Sm3+ in white LED illumination.