The impacts of Li⁺ replacement for Mn²⁺ on the structure, vacancy defects, and magnetic and dielectric characteristics of Mn_1-xLi_xWO₄ (x = 0.00–0.20) ceramics synthesized by solid-state reaction were investigated. The analysis of XRD and Raman results revealed that Mn_1-xLi_xWO₄ samples had single-phase monoclinic structure, and Li⁺ ions had partially replaced Mn²⁺ ions and induced lattice deformation and substitutional disorder in Mn_1-xLi_xWO₄. Positron annihilation lifetime spectroscopy indicated that Li⁺ replacement resulted in the reduction of vacancy size and the increase of vacancy concentration. The dielectric properties measurements showed that Li⁺ replacement significantly enhanced the dielectric constant of the Mn_1-xLi_xWO₄ samples. It could be observed from the magnetic properties measurements that Li⁺ replacement suppressed the AF1 phase, extended the existing temperature range of AF2 phase, and weaken the magnetization of Mn_1-xLi_xWO₄ system. The evolution of magnetic properties was related to vacancy defects, lattice distortion, chemical disorder, and magnetic dilution effect caused by Li⁺ replacement.