Mushroom (Agaricus bisporus) production generates about 0.615 kg CO₂-eq per kg, far below most animal proteins, and using spent substrate cuts emissions by ~38.8% while reducing freshwater pollution by ~91.7%. At the meal level, replacing meat with mushrooms lowers dish emissions by 40%+, with fully mushroom-based meals consistently falling below the 0.31–0.57 kg CO₂-eq planetary boundary per meal. Mushrooms also operate as a circular system, where reusing growing material reduces total environmental impact by ~40%, making them one of the most efficient foods across both production and consumption stages.