Near-infrared (NIR) emitting phosphors with persistent luminescence have attracted much interest in the past decade, due to their widespread applications in medical diagnostics, night-vision surveillance, multi-level anti-counterfeiting, and security encryption. Although NIR luminescent materials have been extensively investigated, it is challenging to explore novel phosphors or new compositions with tunable NIR persistent luminescence performance. Herein, chemical unit co-substitution is explored in a series of NIR emitting Zn_1−2xLi_xGa_2+xO₄:Cr³⁺ solid solutions. Substitution with [Li⁺–Ga³⁺] for the [Zn²⁺–Zn²⁺] unit results in tunable emission in the deep-red and NIR regions coming from the typical Cr³⁺ emission of the ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ transition. Upon UV or X-ray irradiation, intense NIR persistent luminescence is realized and composition-related variation of trap distributions and densities are identified. Importantly, a higher trap density of the Cr³⁺-doped solid solutions (i.e., x = 0.2–0.4) than that of the benchmark NIR persistent phosphor of ZnGa₂O₄:Cr³⁺ and LiGa₅O₈:Cr³⁺ is achieved. This work proves the feasibility of chemical unit co-substitution in developing new NIR persistent phosphors, which can motivate further exploration of luminescent materials with novel properties.