Significance Our work has revealed the nature of local chemical order and has established its significant relationship to the intrinsic and extrinsic stacking fault energy in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. These findings highlight the general need to discern how critical the role of local chemical ordering is in dictating the mechanical of properties of medium- and high-entropy alloys. Our results highlight the possibility of tuning order in disorder to ultimately achieve the science-based design and optimization of new high-entropy alloy systems with specifically desired combinations of macroscale mechanical properties.