The control of the photoluminescence of manganese ions (Mn²⁺ or Mn⁴⁺) is not only fundamental but also a practical issue for the discovery of advanced optical materials. However, the correlation between the valence preference of Mn and structural/composition variation remains challenging. Herein, we have reported an Mn-doped non-stoichiometric Ba_0.75Al₁₁O_17.25 (BAL) compound, in which the unconventional spontaneous reduction of Mn⁴⁺ to Mn²⁺ was observed. The photoluminescence tuning of Mn²⁺/Mn⁴⁺ in Mn-doped Ba_0.75Al₁₁O_17.25–BaMgAl₁₀O₁₇ (BAL–BAM) solid solutions are demonstrated, and their valence state controlling mechanism is proposed based on the defect structure and composition variation. The narrow-band phosphor BAL:Mn as a green light component was applied to fabricate the LED device with a wide color gamut of 107.3% National Television Standards Committee (NTSC) standard. Furthermore, the fluorescence intensity ratio of Mn⁴⁺ to Mn²⁺ in BAM:Mn displayed good temperature sensing capability and repeatability with a maximum relative temperature sensitivity S_r of 1.63% K⁻¹ at 453.15 K (180 °C). This study provides a new insight into the Mn valence state control and photoluminescence tuning for advanced optical applications.