Cellulose nanocrystals (CNCs) are widely used as reinforcing fillers in polymers due to their exceptionally high stiffness and strength and because the biological species from which they are isolated represent renewable resources. However, aggregation of the CNCs, which is concomitant with limited reinforcement, is often difficult to avoid. One-component nanocomposites (OCNs) based on polymer-grafted nanoparticles can solve this problem because this approach affords, by design, materials in which no such aggregation is possible. At the same time, chain entanglements between the CNC-grafted polymer chains provide stress transfer among the particles. To demonstrate this, we investigated OCNs based on polymethacrylate-grafted CNCs. A previously unaccessed compositional space, that is, OCNs with a CNC content of 10 or 20 wt %, was explored. Cotton linter-based CNCs were modified via surface-photoinitiated free radical polymerization, which involved the functionalization of the CNC surfaces with benzophenone moieties as photoradical initiator species and the subsequent surface-photoinitiated polymerization of methyl or hexyl methacrylate under UV irradiation at 365 nm. The resulting particles readily dispersed in THF. Solvent-casting and compression-molding afforded films of homogeneous appearance, which display remarkable improvements in stiffness or toughness and strength in comparison to conventional two-component nanocomposites of unmodified CNCs and the respective polymers.