Cubic phase Li7La3Zr2O12 (LLZO), a member of the Li–Garnet family, is a promising solid electrolyte and has been widely studied in recent years. However, LLZO samples prepared via conventional ambient air sintering reported in the published literature often contain large grains with lower than desired (<94%) relative density. In this study, a non-contact method of co-firing with mother powder method is proposed to prepare high-quality Ta-doped LLZO–MgO composite ceramics. By sintering at 1150°C for 5 h, the ceramics can reach relative density of 98.2%, conductivity of 5.17 × 10−4 S cm−1 at 25 °C and fracture strength of ∼150 MPa. The sintered samples have uniform fine-grained microstructure and high critical current densities of 0.75–0.95 mA cm−2 at room temperature in Li–Li symmetry cell with Au modification. In addition, systematic sintering experiments and characterizations are conducted to explore the function of MgO in inhibiting the Ta-LLZO grain growth and its existing form inside the composite ceramics.