Garnet-type Li₇La₃Zr₂O₁₂ is a promising solid-state electrolyte for lithium (Li)-ion batteries, and it readily reacts with H₂O and CO₂ when exposed to ambient air. Such reactions form a contamination layer on Li₇La₃Zr₂O₁₂, which is detrimental to the battery operation. Here, using first-principles calculations, we investigate the adsorption and reactions of H₂O and CO₂ on a Li₇La₃Zr₂O₁₂ surface. We show that H₂O reacts through the exchange of protons and Li⁺ and produces metal hydroxide species. At high H₂O coverage, half of the H₂O molecules dissociate, while the other half remain intact. CO₂ reacts with the Li₇La₃Zr₂O₁₂ surface directly to produce carbonate species. We clarify that the individual reactions of H₂O and CO₂ with Li₇La₃Zr₂O₁₂ are thermodynamically more favorable than the co-adsorption of H₂O and CO₂. Finally, we demonstrate that low temperature and high partial pressure promote the reactions of H₂O and CO₂ with Li₇La₃Zr₂O₁₂. Our study provides guidance on processing conditions for the garnet Li₇La₃Zr₂O₁₂ solid electrolyte to minimize its surface contamination.