Here, the piezoresistive behavior and the underlying sensing mechanism of carbon nanotubes (CNTs) and cellulose nanocrystals (CNCs) composite strain sensors were studied. Aqueous CNT/CNC inks were developed, characterized, and applied on the surface of glass fiber reinforced polymer (GFRP) composites to function as strain sensors. The sensor's behavior and sensitivities at small and large strains depended on the initial composition of CNCs. The sensor with a CNT/CNC composite with composition of 0.8:0.4 (wt%:wt%) had gauge factors of 0.9 and 6.4 at 0.60 % and 1.35 % strains, respectively. At higher composition of 0.8:4.0 (wt%:wt%), gauge factors of 0.5 and 22.2 were calculated for the same strain regions. Through analytical model and morphology analyses, we discussed the influence of CNCs on CNT contact types and on the average tunneling distance between CNTs and the their piezoresistive performance. It was also discussed that CNC particles control the types of contacts between adjacent CNTs. As a result, tailoring piezoresistive behavior was demonstrated. In conclusion, applying a binder-free and environmentally friendly sustainable aqueous ink on a surface of a composite was revealed to be an effective and practical approach for tailored strain sensing.