Atom transfer radical polymerization is a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Here, we show that an externally applied electrochemical potential can reversibly activate the copper catalyst for this process by a one-electron reduction of an initially added air-stable cupric species (Cu-II/Ligand). Modulation of polymerization kinetics is thereby tunable in real time by varying the magnitude of applied potential. Application of multistep intermittent potentials successfully triggers initiation of polymerization and subsequently toggles the polymerization between dormant and active states in a living manner. Catalyst concentrations down to 50 parts per million are demonstrated to maintain polymerization control manifested in linear first-order kinetics, a linear increase in polymer molecular weight with monomer conversion, and narrow polymer molecular weight distributions over a range of applied potentials.