To meet the growing demand for stable dual-emission phosphors for use in optoelectronic applications, this study investigated a self-reduction strategy with Mn-doped Li₂ZnGe₃O₈ (LZGO) phosphors. The spinel-structured LZGO lattice enables the coexistence of Mn²⁺ and Mn⁴⁺ via oxygen vacancies and lattice defects, achieving visible (Vis) and near-infrared (NIR) dual emission without the need for external reducing agents. Spectroscopic analyses, including X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS), confirmed the presence of heterovalent Mn states, with lifetimes of 3.63 ms (Mn²⁺) and 0.32 ms (Mn⁴⁺) under selective excitation. The LZGO:xMn system thus demonstrates excitation-tunable Vis-NIR luminescence and high stability, making it a cost-effective and environmentally friendly candidate for anticounterfeiting and bioimaging applications. This work presents a defect engineering-driven design concept for developing multifunctional redox-active phosphors with broad application prospects.