Carboxylated cellulose nanofibrils (CNFs) have emerged as effective green dispersants for monolayer montmorillonite (MMT) dispersions. However, their dispersion capability is sensitive to the metal ion concentration in aqueous solutions. Hence, this study investigated the effects of Na⁺ and Ca²⁺ concentrations on the dispersive ability of carboxylated CNFs for monolayer MMTs in water. Quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) were utilized to explore the interfacial interactions between the carboxylated CNFs and monolayer MMTs under different Na⁺ and Ca²⁺ concentrations. When the concentration of Na⁺ reached 0.1 mmol/L, the adhesion mass of carboxylated CNFs on MMT-coated wafer peaked at 24.47 mg/m², higher than control sample (carboxylated CNF-dispersed monolayer MMT dispersion without metal ions, 16.03 mg/m²). Moreover, the electrostatic shielding effect promoted a better dispersion of monolayer MMTs by carboxylated CNF dispersant. With a further increase in the Na⁺ concentration, the surface charge of CNFs and MMTs would be reversed resulting from the improved electrostatic shielding effect, which weaken the dispersive ability of carboxylated CNFs. The addition of Ca²⁺ reduced the dispersive ability of carboxylated CNFs for monolayer MMTs, because Ca²⁺ required a lower concentration for the onset of charge reversal compared to Na⁺. This study provides interfacial scale insights into the influence of metal ion concentration on carboxylated CNF-dispersed monolayer MMT dispersions. It also provides a strategy to enhance the dispersive ability of carboxylated CNFs for monolayer MMTs.