Abstract Cellulose nanocrystals are an important class of bio‐based crystalline nanostructures, finding application in several technological fields, including paper and textile coating, biocomposite engineering, biocatalysts immobilization, etc. This study explores enzymatic hydrolysis of Avicel, using endoglucanase from Aspergillus niger , to find an environmentally friendly method to extract cellulose nanocrystals from cellulose sources. Enzymatic hydrolysis has the advantage of reduced energy consumption and higher environmental friendliness compared to acid hydrolysis. In this work, we report for the first time very high nanocrystals yield by combining mechanical pretreatment of the cellulose starting material with a ball miller and endoglucanase hydrolysis, as a result of an extensive optimization of reaction conditions. In particular, a ball milling pretreatment carried out for 50 minutes at 3 Hz, allowed to isolate enzymatic CNCs with 76 % yield and with crystallinity as high as 75 %. The materials were characterized by X‐Ray diffractometry, attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, dynamic light scattering, zeta potential and field emission scanning electron microscopy (FE‐SEM). Their characteristics were compared with the properties of sulfated CNCs, prepared from Avicel by sulfuric acid hydrolysis. Our results are technologically relevant, as they contribute to the accessibility and sustainability of CNCs for a wide range of applications in various industries.