The electrochromic (EC) behavior of tungsten oxide (WO₃) is strongly influenced by its nanoscale architecture and local chemical environment, which determine the efficiency of ion–electron transport during switching. Here, a systematic polyvinylpyrrolidone (PVP)–Manganese (Mn) synergistic engineering approach was developed using bare WO₃ (WO), PVP-assisted WO₃ (P-WO), Mn-doped WO₃ (Mn-WO), and PVP-assisted Mn-doped WO₃ (P-Mn-WO) to clarify the individual and combined effects of PVP and Mn on EC performance. All films retained the amorphous WO₃ structure, while morphological evolution revealed a transition from compact nanoparticle assemblies to an interconnected porous architecture in P-Mn-WO. The optimized P-Mn-WO film exhibited enhanced Li⁺ transport, with diffusion coefficients of 0.45 × 10⁻¹⁰ and 4.08 × 10⁻¹⁰ cm²/s for oxidation and reduction, respectively. This favorable transport environment enabled 75.8% optical modulation at 635 nm, a coloration efficiency of 142.63 cm²/C, and 92.59% charge reversibility, with only 5.24% degradation during repeated switching. At the device level, P-Mn-WO delivered 82.30% bleached transmittance, 12.02% colored transmittance, and 70.28% optical modulation, while showing only 4.5% degradation over 2000 s of continuous operation. These results demonstrate that coupling PVP-assisted growth with Mn doping provides effective control over WO₃ morphology, Li⁺ transport, charge utilization, and optical switching, establishing a promising route toward a durable, high-contrast WO₃-based EC smart-window.