Surface hardening by mechanical attrition at room and cryogenic temperatures has been studied on CrMnFeCoNi high- and CrCoNi medium-entropy alloys. The hardness gradient is produced by severe plastic deformation through dislocation slip and mechanical twinning, finally leading to an ultrafine or even nanocrystalline structure. The hardness of samples after surface attrition at cryogenic temperature is slightly less than that of samples deformed at room temperature. A small amount of deformation-induced martensitic transformation is only observed in a narrow surface layer of the medium-entropy alloy severely deformed under cryogenic conditions. Reasons for these observations are discussed. R.C. acknowledges Project No. 2020/37/B/ST5/03267 of the National Science Centre of Poland. This study was also supported in part by the French State through the program “Investment in the future” operated by the National Research Agency (ANR) and referenced under No. ANR-11-LABX-0008-01 (Labex DAMAS).