Integral cement sheaths are crucial for safe and economical hydrocarbon production throughout the well lifecycle. Cement additives tailor short- and long-term cement properties for various well conditions to ensure well integrity. Although various additives exist, the current trend in reducing the carbon footprint motivates the developing "greener” additives that are environmentally friendly and made from renewable and sustainable sources such as cellulose nanocrystals (CNC). CNCs exhibit superior properties and have shown significant impact on cement slurry, including increased degree of hydration, strength, and altered properties. However, most studies on CNCs are intended for construction industry rather than hydrocarbon and geothermal well cementing. Investigating the use of CNCs as high-performance cement additives is therefore of interest due to their potential benefits. This study aims to determine the effect of CNC on vital well cement properties. The effects of CNC were determined using standard American Petroleum Institute (API) test procedures and equipment in an experimental approach. The experimental findings indicate that the addition of cellulose nanocrystals (CNC) at a concentration of 2 vol% resulted in a notable increase of 7% in viscosity, a significant decrease of 50% in free water, a remarkable reduction of 78% in cement shrinkage, and no discernible effect on slurry thickening time. Furthermore, the inclusion of a 0.2 vol% of CNC yielded a significant surge of 56% in compressive strength after 21 days and accelerated 500 psi strength development by 9%. However, the investigation revealed that a concentration of 1.5 vol% of CNC represents a threshold concentration or the turning point, beyond which the addition of CNC can negatively impact the studied cement properties.