Zn₂GeO₄:Mn²⁺ nanophosphors are synthesized from germanium(IV) oxide and acetates of zinc and manganese(II) in a mixed solvent of water and diethylene glycol (DEG) by the solvothermal reaction at 200 °C for 2 h. Phase pure Zn₂GeO₄:Mn²⁺ is obtained for the samples prepared at 0 ≤ x_DEG ≤ 91.7, where x_DEG is the volume percentage of DEG in the mixed solvent. The particle and crystallite sizes decrease with the increase in x_DEG. The sample prepared at x_DEG = 91.7 comprises the nanorods with 30.2 and 12.2 nm in mean length and width, respectively. The actual Mn concentration measured by X-ray fluorescence analysis increases with the increase in x_DEG and approaches to the nominal concentration, 2.0 at %. The samples show green luminescence corresponding to the d−d transition of Mn²⁺ under the irradiation of UV and near-UV light. The photoluminescence intensity reaches a maximum around x_DEG ∼ 80. This is attributed to the competitive factors, the Mn concentration and the sizes of crystallites and particles.