Doubly ordered double perovskites with AAʹBBʹO₆ stoichiometry are promising new host materials for Eu³⁺ activator ions. In this paper, double perovskite (Na,Li)YMg(W,Mo)O₆:Eu³⁺ phosphors were synthesized by a high-temperature solid-state method and the effect of cation substitutions in this series of monoclinic-type NaYMgWO₆ compounds was investigated in order to control the photoluminescence properties. The composition modulation and structural evolution of (Na,Li)YMg(W,Mo)O₆:Eu³⁺ were studied by powder X-ray diffraction and Rietveld refinement. The microstructure and composition of the phosphor materials were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. All compounds' photoluminescence properties were measured, and the luminescence properties were related to the structural properties of the materials. The (Na,Li)YMg(W,Mo)O₆:Eu³⁺ phosphors emit intense red light dominated by the ⁵D₀→⁷F₂ transition at ∼618 nm, along with other characteristic transitions from Eu³⁺ 4f-4f transitions when excited at ∼395 nm and ∼466 nm. The red emission intensity of Eu³⁺ reaches a maximum for LiYMgW_0.9Mo_0.1O₆:Eu³⁺ phosphor, which exhibits high quantum efficiency and excellent thermal stability. The obtained results suggest that these new double perovskites doped with Eu³⁺ are promising red phosphor candidates for white light-emitting diodes.