Extracellular electron transfer (EET) is crucial in microbial energy-conversion technologies. However, broad application is hindered by insufficient charge transfer from microbes to electrodes. Fumarate, although should theoretically inhibit EET as a competing electron acceptor, was shown to moderately enhance EET in Shewanella oneidensis MR-1. In this work, a 50-fold increase in EET currents in the presence of 30 mM fumarate is demonstrated, leading to a 100-fold reduction in electrical resistance to biocurrents based on electrochemical impedance spectroscopy analysis. A fast decrease in currents following the depletion of fumarate and a rapid increase upon reintroducing fumarate revealed hitherto unreported EET dynamics. Through enzymatic assays, the ratio of electrons channeled from lactate metabolism into fumarate reduction and EET, and the concentrations of fumarate necessary for days-long high EET are determined. These new aspects promise to contribute to the development of more efficient microbial technologies without employing any genetic and even materials modifications.