Lithium solid electrolytes are a promising platform for achieving high energy density, long-lasting, and safe rechargeable batteries, which could have widespread societal impact. In particular, the ceramic oxide garnet Li7La3Zr2O12 (LLZO) has been shown to be a promising electrolyte due to its stability and high ionic conductivity. Two major challenges for commercialization are the manufacture of thin layers and the creation of stable, low-impedance interfaces with both anode and cathode materials. Atomic layer deposition (ALD) has recently been shown to be a powerful method for depositing both solid electrolytes and interfacial layers to improve the stability and performance at electrode–electrolyte interfaces in battery systems. Herein, we present a thermal ALD process for LLZO, demonstrating the ability to tune composition within the amorphous as-deposited film, which is studied using in situ quartz crystal microbalance measurements. Post annealing using a variety of substrates and gas environments was performed, and the formation of the cubic phase was observed at temperatures as low as 555 °C, significantly lower than what is required for bulk processing. Additionally, challenges associated with achieving a dense garnet phase due to substrate reactivity, morphology changes, and Li loss under the necessary high-temperature annealing are quantified via in situ synchrotron X-ray diffraction.