Graphene's sp^2-bonded carbon lattice makes it exceptionally resistant to covalent functionalization, with existing approaches relying on chemically specific and often harsh methods, fragmenting our understanding of its reactivity. Here we show that electric fields of approximately 1 V nm^-1 in non-aqueous electrolytes enable the electrochemical functionalization of monolayer graphene with a broad range of substituents, leading to two-order-of-magnitude conductance modulation stable over 5,000 cycles and semi-reversible electronic conductor-insulator transitions. Analysis of the oxidation potentials across substituents reveals trends governed by classical descriptors. Species forming localised σ bonds display increasing oxidation potentials with the meta-Hammett constant (σ_m), whereas substituents interacting via delocalised π orbitals follow an inverse dependence on the modified Swain-Lupton resonance parameter (R). These relationships indicate that graphene's oxidation potential increases with the electron-withdrawing strength of the substituent. Our results provide new insights into the electrochemical activity of inert materials under strong electric fields.