This work successfully fabricated flexible substrates made of cellulose nanocrystal-reinforced chitosan (CNC-Ch) through a simple water-based casting method. The rice husk cellulose nanocrystals (CNCs) were employed to reinforce the chitosan (Ch) matrix in the range of 10–50 wt%. The CNC-Ch substrates produced compact homogenous film with good dispersion and distribution of CNCs within the chitosan matrix. Afterward, the CNC-Ch substrate was applied to form a self-standing hard carbon anode with a mass loading of 5 mg/cm2. As an anode, the hard carbon with 10 wt% of CNC showed the highest electronic conductivity of 0.9774 S/cm and ionic conductivity of 2.12 × 10–5 S/cm in half-cell of hard carbon | NaClO4 1 M EC: DMC (1:1) | Na, with acceptable mechanical properties. These characteristics indicate a promising CNC-Ch substrate candidate for flexible hard carbon anodes for advanced sodium-ion battery applications.