A critical parameter for the large-scale integration of solid-state batteries is to establish processing strategies to assemble battery materials at the lowest processing temperature possible while keeping lithium conduction up. Despite extensive research efforts, integrating ceramic film electrolytes while keeping a high lithium concentration and conduction at a low processing temperature remains challenging. Here, we report an alternative ceramic processing strategy through the evolution of multilayers establishing lithium reservoirs directly in lithium–garnet films that allow for lithiated and fast-conducting cubic solid-state battery electrolytes at unusually low processing temperatures. A lithium–garnet film processed via the multilayer processing approach exhibited the fastest ionic conductivity of 2.9 ± 0.05 × 10−5 S cm−1 (at room temperature) and the desired cubic phase, but was stabilized at a processing temperature lowered by 400 °C. This method enables future solid-state battery architectures with more room for cathode volumes by design, and reduces the processing temperature.