Ionic strength sensing is extremely useful in numerous fields, in particular portable and robust sensors with rapid and precise detection ability are urgently needed. This study presents a novel nanocellulose composite filament sensor with exceptional accuracy (R2 > 0.998) in detecting ionic strength using only trace amounts (20 μL) of liquid samples. The fabrication process involves two straightforward steps: (1) integration of raw carbon nanotubes (CNTs) into a spinning dispersion based on cellulose nanofibrils (CNFs) through a facile co-grinding method; and (2) production of composite filaments via wet-spinning to utilize the assembling structures by these two fibrillar nanoparticles, providing superior mechanical properties and baseline conductivity. These composite filaments confine the capillary swelling within nano-channels, enabling controlled ion diffusion and precise measurement capabilities for a wide range of ionic strengths ranging from 10−5 to 10−1 M within 1 minute. The superior selectivity towards different ions, ability to precisely determine ion content for purified water quality, anti-disturbance capacity, and performance stability under varying environmental conditions are examined. Overall, this study provide a new insight of constructing   carbohydrate and carbon materials at nano- and macro-levels, which greatly boosts the potential of nanocomposites for practical sensor applications.