<p>This work presents a comprehensive first-principles study of the structural, electronic, optical, elastic, and magnetic properties of the halide double perovskites Rb₂MnYF₆ (Y = Ag, Cu) using density functional theory within the full-potential linearized augmented plane wave (FP-LAPW) approach. Geometry optimization confirms a cubic Fm-3&#xa0;m symmetry, with lattice parameters of 8.6528 Å for Rb₂MnAgF₆ and 8.2001 Å for Rb₂MnCuF₆. The calculated bulk moduli, 64.72 GPa and 156.10 GPa, indicate significantly higher incompressibility in the Cu-based compound. Spin-polarized electronic band structures show that both Rb₂MnCuF₆ and Rb₂MnAgF₆ possess half-metallic character, with metallic bands crossing the Fermi level in the spin-up channel and a finite band gap in the spin-down channel. This complete spin polarization confirms their suitability for spintronic applications. Optical simulations indicate strong absorption in the visible range (2–2.5&#xa0;eV), enhanced UV reflectivity for Rb₂MnAgF₆, and distinct composition-dependent variations in dielectric behavior and optical conductivity. Elastic analysis confirms mechanical stability for Rb₂MnAgF₆ (C₁₁ = 125.61 GPa, C₁₂pt&gt; = 31.62 GPa, C₄₄pt&gt; = 30.94 GPa), whereas Rb₂MnCuF₆ exhibits elastic instability due to a negative shear modulus (C₄₄pt&gt; &lt; 0). Magnetic properties are dominated by Mn²⁺ ions, with total magnetic moments of 4.00 µB for Rb₂MnAgF₆ and 4.40 µB for Rb₂MnCuF₆. Overall, Rb₂MnAgF₆ emerges as a mechanically stable, optically active, and magnetically ordered material suitable for multifunctional spintronic and optoelectronic applications, while Rb₂MnCuF₆ requires structural stabilization for practical deployment.</p>