The development of cellulose-based nanomaterials with tailored surface properties is essential for advancing their integration into high-performance polymer nanocomposite systems. In this work, a mechanochemical strategy for the simultaneous production and surface functionalization of cellulose nanofibrils (CNFs) via wet ball milling, employing citric acid as a green crosslinking catalyst and N-alkanals (octanal, heptanal, and their binary mixture) as hydrophobizing agents, is presented. This one-pot process enables in situ surface modification while preserving the nanofibrillar structure of cellulose, providing a scalable, environmentally friendly method for the fabrication of functional nanofillers. Commercial microcrystalline cellulose was milled in an aqueous medium using high-energy planetary ball milling for an optimized time of 20 minutes. The resulting nanofibrils exhibited diameters below 100 nm and improved surface hydrophobicity, confirmed by increased static contact angle measurements. The modified CNFs also displayed stable dispersion behavior and were fabricated into thin films using vacuum-assisted self-assembly. Optical microscopy in reflected light mode revealed morphological differences linked to the specific N-alkanal used, indicating tunable surface texturing. This dual-function approach not only simplifies nanocellulose processing but also enables the engineering of interface-active nanomaterials suitable for reinforcing biodegradable polymer matrices. The resulting nanocellulose systems demonstrate strong potential for use in advanced nanocomposite applications, including sustainable packaging, coatings, and barrier materials.