The covalent modification of large-area graphene sheets by p-(N-Maleimido)phenyl (p-MP) via electrochemical grafting of p-(N-Maleimido)benzenediazonium tetrafluoroborate (p-MBDT) is successfully demonstrated for the first time. The deposition process is monitored in-situ using the mass change of a graphene/SiNX:H/Au-coated quartz crystal microbalance(QCM) chip. The resulting mass increase correlates with a maleimide thickness of approximately 2.3 molecular layers. The presence of an infrared absorption band at 1726 cm-1 shows that maleimide groups were deposited on the substrates. Raman backscattering spectra reveal the presence of D and D′ modes of the graphene layer, indicating that p-MP forms covalent bonds to graphene. Using the mass change and charge transfer during the potential cycling the faradaic efficiency of the functionalisation process was deduced, which amounts to eta = 22%.