Carboxymethylated cellulose nanofibril (CMCNF) is an effective green dispersant to prepare well-dispersed monolayer montmorillonites (MMTs) in water, thereby facilitating the preparation of a high-performance MMT/polymer nanocomposite film. However, not enough attention has been paid to correlating the degree of substitution (DS) of CMCNFs with the mechanical and optical properties of the final nanocomposite films. In this study, a series of homogeneous monolayer MMT nanoplatelet dispersions was prepared initially using CMCNFs with different DS as a dispersant, and the as-prepared CMCNF-dispersed MMT dispersions were then mixed with sodium carboxymethyl cellulose (CMC-Na) to fabricate nacre-like nanocomposite films with different contents of MMTs through self-assembly. The layered nanostructure and optical and mechanical properties of the as-prepared CMCNF-dispersed MMT/CMC-Na nanocomposite films were investigated, which demonstrated that CMCNFs with lower DS have a positive effect on their optical and mechanical properties. This study sheds light on the preparation of MMT-based nanocomposite films with superior optical and mechanical properties.