Owing to its low cost and admirable luminescent characteristics for use in warm white-light-emitting diode (w-LED) applications, the non-rare-earth Mn4+-activated red phosphor has emerged as a potent competitor of commercial Eu2+-doped nitrides in recent years. In this work, the novel red-emitting phosphor BaMgAl10–2xO17:xMn4+,xMg2+ is successfully synthesized, which exhibits bright and narrow-band luminescence peaking at 660 nm with a full width at half-maximum of merely ∼30 nm upon blue light excitation. The unique structural feature of BMA, i.e., alternating arrangements of Mn4+-doped MgAl10O16/undoped BaO layers in the z direction and Mn4+-doped [AlO6]/undoped [AlO4] groups in the x–y plane, favors efficient Mn4+ luminescence by reducing nonradiative energy loss channels. Unlike previously reported hosts, BMA accommodates Mg2+ in the lattice without destabilizing the crystal structure. Remarkably, partitioning Mg2+ in the host not only greatly enhances Mn4+ luminescence by 1.84-fold but also retards the concentration quenching effect induced by Mn4+ dipole–dipole interactions owing to the reduced number of Mn4+–Mn4+–O2– pairs. Spectroscopy demonstrates that the luminescence of optimized BMA:0.02Mn4+,0.02Mg2+ exhibits a high color purity of 98.3%, good color stability against heat, and excellent resistance to thermal impact. When incorporating BMA:0.02Mn4+,0.02Mg2+ and YAG:Ce3+ phosphors into an oxide glass matrix at various ratios and then coupling the phosphor-in-glass color converters using a blue chip, the chromaticity parameters of the fabricated w-LED are well-tuned, with the correlated color temperature decreasing from 6608 to 3622 K and the color rendering index increasing from 68.4 to 86.0, meeting the requirements for in-door lighting use.