Hydrophobic polymer-grafted cellulose nanocrystals (CNCs) were produced surface-initiated atom-transfer radical polymerization (SI-ATRP) in two different solvents to examine the role of reaction media on the extent of surface modification. Poly(butyl acrylate)-grafted CNCs were synthesized in either dimethylformamide (DMF) (D-PBA--CNCs) or toluene (T-PBA--CNCs) alongside a free polymer from a sacrificial initiator. The colloidal stability of unmodified CNCs, initiator-modified CNCs, and PBA--CNCs in water, DMF, and toluene was evaluated by optical transmittance. The enhanced colloidal stability of initiator-modified CNCs in DMF led to improved accessibility to initiator groups during polymer grafting; D-PBA--CNCs had 30 times more grafted chains than T-PBA--CNCs, determined by thermogravimetric and elemental analysis. D-PBA--CNCs dispersed well in toluene and were hydrophobic with a water contact angle of 124° (for polymer grafts > 13 kDa) compared to 25° for T-PBA--CNCs. The cellulose crystal structure was preserved, and individual nanoparticles were retained when grafting was carried out in either solvent. This work highlights that optimizing CNC colloidal stability prior to grafting is more crucial than solvent-polymer compatibility to obtain high graft densities and highly hydrophobic CNCs SI-ATRP.