The Mn4+ activated fluostannate Na2SnF6 red phosphor was synthesized from starting materials metallic tin shots, NaF, and K2MnF6 in HF solution at room temperature by a two-step method. The formation mechanism responsible for preparing Na2SnF6:Mn4+ (NSF:Mn) has been investigated. The influences of synthetic parameters: such as concentrations of HF and K2MnF6 in reaction system, reaction time, and temperature on crystallinity, microstructure, and luminescence intensity of NSF:Mn have been investigated based on detailed experimental results. The actual doping concentration of Mn4+ in the NSF:Mn host lattice is less than 0.12 mol%. The most of K2MnF6 is decomposed in HF solution especially in hydrothermal system at elevated temperatures. The color of the as-prepared NSF:Mn samples changes from orange to white when the temperature is higher than 120 degrees C, which indicates the lower concentration of luminescence centers in the crystals. A series of warm white light-emitting diodes with color rendering index (CRI) higher than 88 and correlated color temperatures between 3146 and 5172 K were obtained by encapsulating the as-prepared red phosphors NSF:Mn with yellow one Y3Al5O12:Ce3+ (YAG:Ce) on 450 nm blue InGaN chips. The advantage of the synthetic strategy to obtain NSF:Mn can be extended to developing Mn4+-doped red phosphors from low-costing metals at room temperature for large-scale industrial applications.