It becomes ever more evident that cognitive operations serve as fundamental mechanisms underlying higher order ruminative thoughts. In this sham controlled within subjects study, we performed anodal transcranial Direct Current Stimulation (tDCS) over the left dorsolateral prefrontal cortex (DLPFC) in 32 healthy participants. We tested the causal hypothesis that the relationship between DLPFC activity and ruminative thinking is mediated by working memory operations. We used the Internal Shift Task, a paradigm in which participants have to update and shift between specific (non)emotional representations in working memory. Subsequently, during an unguided rest period approximately 20 min after the stimulation, we explored the occurrence of momentary ruminative self-referent thought. The results demonstrated that the influence of anodal tDCS of the left DLPFC (and not of sham stimulation) on momentary ruminative self-referent thinking is mediated by the enhancement of WM operations for angry faces. Moreover, the more individuals ruminate in everyday life (as measured using the Ruminative Response Style), the larger this mediation effect was. These findings suggest that enhancing cognitive self-regulation, by increasing the ability to update and shift away from negative representations in working memory, might help individuals to control unintentional streams of self-referent thoughts that are self-critical and self-evaluative, a thinking style known as rumination.