It is extremely necessary to develop LEDs with a high color index for many special applications. In this study, novel single-phase warm white light-emitting 3/2-mullite (Al₆Si₂O₁₃):Eu²⁺,Mn²⁺ phosphors were prepared via the high-temperature solid-state reaction method. The emission hue can be precisely controlled from (0.2496, 0.3032) to (0.4001, 0.3031) by altering the Eu²⁺/Mn²⁺ ratio via the energy transfer route, where warm-white emissions with CIE coordinates of (0.3440, 0.3302) are achieved using UV-excited Al₆Si₂O₁₃:2.0%Eu²⁺,0.6%Mn²⁺ phosphors. The energy transfer between Eu²⁺ and Mn²⁺ in the Al₆Si₂O₁₃ host is demonstrated to be a nonradiative dipole–dipole interaction. A warm-white LED device comprising the Al₆Si₂O₁₃:2.0%Eu²⁺,0.6%Mn²⁺ phosphor and an NUV (365 nm) chip operated at 120 mA yields white-light emissions with a correlated color temperature of 5656 K and color coordinates of (0.3289, 0.3413). Moreover, it should be noted that the color rendering indices of R_a and R9 can reach up to 92.4 and 97.0, respectively. Our proposed Al₆Si₂O₁₃:2.0%Eu²⁺,0.6%Mn²⁺ phosphor with high color rendering index has good application prospects.