In order to realise the potential of graphene nanocomposites it is vital to control the degree of dispersion and achieve a strong graphene/polymer interface. Herein, we developed a facile ‘grafting to’ functionalization approach for graphene nanoplatelets. NH2-terminated graphene nanoplatelets (NH2-GNPs) prepared by diazonium coupling were used as a ‘platform’ to covalently graft PMMA chains to the surface of graphene through amidation between the –NH2 groups and PMMA chains (PMMA–NH–GNPs). A degree of PMMA grafting of ∼3.8 wt% (one chain per ∼40 carbon atoms) was found to both improve the dispersion of the GNPs in a PMMA matrix and give strong graphene/polymer interfaces compared to as-provided GNPs. Thus, 2 wt% of PMMA–NH–GNPs in PMMA was found to increase the elastic modulus, strength and strain at break of PMMA, whereas the incorporation of unmodified GNPs showed poor levels of reinforcement at all loadings. Furthermore, the glass transition temperature (Tg)and the onset of decomposition (Td) of PMMA were increased by 15 °C and 29 °C, respectively, by adding 5 wt% of PMMA–NH–GNPs, whereas incorporating unmodified GNPs led to smaller increases. This work offers the possibility of controlling the properties of graphene/polymer composites through chemically tuning the graphene/polymer interface, which will have broad implications in the field of nanocomposites.