Ce3+-, Tb3+-, and Mn2+-activated Ca9MgNa(PO4)7 (CMNP) phosphors have been prepared by a Pechini-type sol–gel method. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) and cathodoluminescence (CL) spectroscopy were used to characterize the samples. The energy transfer from Ce3+ to Tb3+ in CMNP phosphors has been demonstrated to be a resonant type by a dipole–quadrupole mechanism, and the Ce3+ to Tb3+ critical distances (RC) calculated by the concentration-quenching method and the spectral-overlap method are 12.21 and 12.11 Å, respectively. A color-tunable emission in CMNP phosphors can be realized by Ce3+→Tb3+ or Ce3+→Mn2+ energy transfer. White cathodoluminescence has been realized in a single-phase Ca9MgNa(PO4)7 host by codoping with Ce3+, Tb3+, and Mn2+ for the first time with International Commission on Illumination (CIE) chromaticity coordinates (0.319, 0.327). Furthermore, the cathodoluminescence (CL) properties of CMNP:Ce3+/Tb3+/Mn2+ phosphors, including the dependence of the CL intensity on accelerating voltage and filament current, the decay behavior of CL intensity under electron bombardment, and the stability of the CIE chromaticity coordinates, have been investigated in detail. Owing to their good CL properties and good CIE chromaticity coordinates, the as-prepared phosphors have potential application in field emission display (FED) devices.