Li₇La₃Zr_2.0O₁₂ (LLZO) garnets have many properties of a suitable solid electrolyte for lithium ion batteries; however, drawbacks like their low ionic conductivity and poor interfacial properties still hinder their broadened application. Current research has sought to both increase their ionic conductivity and density their microstructure, which are two important factors relating to their practical application. This study presents a facile and effective way to obtain compact LLZO with high Li⁺ conductivity via a one-step multi-elemental doping strategy, using Nb⁵⁺ and Ga³⁺ as dopants simultaneously. Garnet-structured oxides with the nominal chemical composition of Li_6.8−3xGa_xLa₃Zr_1.8Nb_0.2O₁₂ (x = 0, 0.1, 0.15, 0.2, 0.25, and 0.3) are prepared via a conventional solid-state reaction, and the effects of the two dopants on the oxides are investigated based on the phase compositions, morphologies and Li ion conductivities. Benefitting from Li⁺ vacancies generated by the addition of the two elements, garnets sintered with an optimized amount of Ga/Nb dopant can exhibit relative densities of 93–95% and a peak conductivity of 1.42 × 10⁻³ S cm⁻¹ at 50 °C. In a symmetric Li/LLZO/Li cell, this dense structured electrolyte shows a low overpotential and superior electrochemical stability to Li metal, exhibiting good performance for over 200 h at a current density of 100 μA cm⁻¹ during Li plating/stripping cycles. Also, this co-doped solid-state electrolyte can exhibit acceptable cycling stability when paired with a LiNi_0.33Mn_0.33Co_0.33O₂ (NMC111) cathode, both with the help of liquid electrolyte and when assembled as an all-solid-state battery. We believe this research can provide some new insights into developing solid-state electrolyte-based lithium ion batteries.