Obtaining highly efficient photoluminescence with Mn4+-activated phosphors, which have been extensively studied in diverse lighting devices, requires the precise control of the manganese valence states. However, this control is difficult to achieve because manganese ions can have various valence states ranging from divalent to heptavalent. Additionally, the concentrations of Mn ions in each valence state, especially the effective Mn4+ concentration, have never been quantitatively determined in a phosphor crystal lattice. The relationship between the effective Mn4+ concentration and the luminescence properties of Mn4+-activated phosphors is of current interest for improving the phosphor properties. In the present study, the effective Mn4+ concentration in Li2TiO3:Mn4+ (LTO:Mn) phosphors prepared by the sol-gel method with heating at various temperatures was quantitatively analyzed by X-ray absorption near-edge spectroscopy. Moreover, the effect of the existence of Mn2+, Mn3+, and Mn4+ ions on the photoluminescence efficiency was investigated. The effective Mn4+ concentration was found to be over 60% in all phosphor samples. The quantum efficiencies (QEs) of all LTO:Mn phosphors strongly depend on the effective Mn4+ concentration. In particular, the LTO:Mn phosphor prepared by heating at 800 °C (LTO:Mn@800) contained the highest effective Mn4+ concentration of 98.1% and exhibited the highest internal QE of 31.6%. The results of this work provide new and important insights for the development of Mn4+-activated phosphors with high efficiency.