Inhibiting actions when they are no longer appropriate is essential for adaptive goal‐directed behavior. In this study, we used high‐density EEG and a standard flanker task to explore the spatiotemporal dynamics of cognitive control and inhibitory mechanisms aimed to prevent the commission of errors. By recording hand‐related electromyographic activity, we could disentangle successful from unsuccessful inhibition attempts. Our results confirm that (a) the latency of the error‐related negativity (ERN; or Ne) component is too late to be associated with these online inhibitory mechanisms, and (b) instead, a frontal slow negative component with an earlier time course was associated with the implementation of online inhibition. These findings are consistent with single‐cell recordings in monkeys showing that the supplementary motor area provides cognitive control signals to the primary motor cortex to exert online inhibition and in turn rectify the course of erroneous actions.