This study provides an overview of the utilization of cellulosic nanobiopolymers as a multifunctional additive for the mechanical and durability performance enhancement of cementitious composites. The cellulosic nanobiopolymer used in this study consists of cellulose nanofibrils (CNF) with high aspect ratio, high elastic modulus, and reactive surface sites. It was observed that a small dosage (up to 1% by weight of cement) of CNF can enhance the flexural strength and elastic modulus of the paste samples by 47% and 100%, respectively. Due to the presence of negatively charged carboxyl surface sites, the addition of CNF delays the early age cement hydration reaction. Nevertheless, nanomechanical investigations showed that the addition of CNF can increase the relative amounts of high-density C-S-H in the hydrated cement paste matrix. Finally, due to the cation exchange capacity, these nanobiopolymers have shown to suppress alkali-silica reaction (ASR) gel formation when used up to 3% by weight of cement.