Hexagonal boron nitride nanosheets (BNNSs) show great applications in metal protection, thermal management, energy storage, and medicine fields due to their many unique properties. However, the scalable preparation of large-size ultrathin BNNSs still remains a huge challenge to date. This study develops a sustainable mechanochemical exfoliation (SMCE) approach for high-yield (74%) synthesis of BNNSs by polylactide-assisted ball milling from bulk hexagonal boron nitride (hBN). The obtained BNNSs have an ultrahigh aspect ratio of ≈1240 and a low defect degree and show superior disperse ability in liquids. Combining experimental and density functional theory (DFT) results, a reasonable mechanism of edge-selective grafting is proposed. In view of the excellent processability and self-assembly ability of such large-size ultrathin BNNSs, high-performance and transparent BNNSs/epoxy (EP) composites with greatly improved anticorrosion and mechanical properties are then fabricated. Typically, the BNNSs/EP composites show an extremely high protection efficiency of 99.9%, and the tensile strength and modulus increase by 124 and 66%, respectively. We believe that this green and efficient exfoliation route will boost the development of scalable production of large-size ultrathin BNNSs and shows huge promise for their widespread application in functional composites.