Cognitive control is flexible and often based on two competing processing modes: reactive vs. proactive control. This begs the question of how human subjects can flexibly switch between them. In this EEG study, we addressed this question by systematically altering the contingency created between conflict and performance in different conditions. Fifty-eight participants performed the confound-minimized Stroop task wherein either congruent or incongruent trials were selectively reinforced by an evaluative feedback. Behavioral results showed that while reactive control improved the most when congruent trials were reinforced, proactive control actually benefited from the reinforcement of incongruent trials. Interestingly, at the EEG level, we found that proactive control mostly influenced the N450 component, but also increased attention allocated to the stimulus at the occipital P1 level. In comparison, reactive control mostly influenced and boosted evaluative feedback processing at the P3 level. Moreover, we found that although mid-frontal theta was increased by conflict and negative feedback, it did not capture this flexibility in cognitive control, as revealed by these ERP components. These findings suggest that proactive and reactive control can have distinct electrophysiological signatures when specific stimulus and feedback-locked ERP components are considered concurrently.