The development of a multifunctional wood wax oil-based coating holds significant importance in meeting demands for wood protection and enhanced longevity of wood materials. Nevertheless, a simple method of developing such an integrated coating that possesses excellent durability, ultraviolet (UV) resistance, water resistance, and environmental friendliness remains a significant obstacle. In this paper, stearoyl chloride-grafted cellulose nanocrystals (SCNCs) with excellent compatibility with wood wax oil (WWO) coatings were prepared by combining cellulose nanocrystals (CNCs) with stearoyl chloride through a one-step non-homogeneous esterification reaction. Nanocomposite coatings were prepared by a simple one-step ultrasonic miscibility method using SCNCs as modified functionalized fillers. The prepared SCNC-WWO wood coatings had high hardness (grade H), strong adhesion (class 0, with no peeling), high abrasion resistance, good UV resistance, high hydrophobicity (water contact angle of 116°), and good water resistance (maintainable for 1 day). In addition, SCNCs with gas barrier properties can effectively inhibit the release of pollutants within the coatings, making this composite coatings environmentally friendly. Therefore, SCNC-WWO can be used as a high-performance coating for substantial protection of wood in harsh environments, and the strategy for developing it provides insight into the preparation of multifunctional wood paint.