Mn²⁺-doped phosphates Na₂CaMg_1−xMn_x(PO₄)₂ (x = 0.05–1.0) were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), the emission and excitation spectra, and decay measurements were employed to characterize the synthesized phosphors. The XRD patterns show that Na₂CaMg_1−xMn_x(PO₄)₂ (x = 0.05–0.3) forms the single low temperature monoclinic phase α-Na₂CaMg(PO₄)₂ with the crystal group of P2₁/c (No.14). The heavily Mn²⁺-doped Na₂CaMg_1−xMn_x(PO₄)₂ (x = 0.6–1.0) crystallizes in the single high-temperature trigonal phase of β-Na₂CaMg(PO₄)₂ with space-group Pm1 (No. 164). The members with x = 0.3–0.5 form a series of solid solutions containing two distinct phases, α- and β-Na₂CaMg(PO₄)₂. The dependence of luminescence spectra on the Mn²⁺-doping concentration in Na₂CaMg_1−xMn_x(PO₄)₂ (x = 0.05–1.0) was investigated. The great red-shift of Mn²⁺ emission with increasing Mn²⁺-concentration in Na₂CaMg(PO₄)₂ were observed. The results are discussed in relation with the detailed crystal structure and the spectral analyses. The CIE coordinates and the luminescence decay (lifetimes) of Mn²⁺ ions were discussed in order to further investigate the potential applications.