A series of APF6:Mn4+ (A = Cs+, Rb+, K+) phosphors with single phases were obtained via a simple ion exchange method. These phosphors can exhibit red emission with a broad excitation band in the blue region. The optimized concentration of Mn4+ in APF6:Mn4+ was determined. The Racah parameters B and C of Mn4+ in APF6:Mn4+ were also calculated. The results indicate that Mn4+ in CsPF6:Mn4+ experiences the weakest crystal field among APF6:Mn4+. The phosphor CsPF6:Mn4+ prepared with the HPF6/K2MnF6 molar ratio of 10 : 1 has the strongest red emission and the highest external quantum efficiency of 50.4% among the samples. It has high thermal quenching resistance and its emission intensity at 160 °C is about 1.1 times higher than that at room temperature. This sample also can remarkably improve the color rendering index and decrease the colour rendering index of white light-emitting diodes (w-LEDs). Hence, CsPF6:Mn4+ could be used as a red-emitting phosphor for warm w-LEDs.