Lattice defect plays a significant role in the optical properties of elastic mechanoluminescent materials, which could be modulated by cationic non-equivalent replacement. Here, a series of novel mechanoluminescent phosphors Li2-xMgGeO4:xMn2+ (0 ≤ x ≤ 0.025) were synthesized via a high-temperature solid-state reaction method in an ambient atmosphere. The defect type and its relationship with optical perfomance were clarified via X-ray photoelectron spectroscopy, electron spin resonance, and thermoluminescent spectroscopy. Along with the introduction of Mn ions, the trap levels of oxygen vacancies become shallow, which are beneficial to produce long afterglow and mechanoluminescence. This study offers a feasible approach for developing new functional materials via defect control in self-reduction systems.