Cognitive control can be triggered by explicit or implicit events; it has been proposed that these two possibilities tap into dissociable mechanisms. In this study, we investigate this idea by testing whether young children, who struggle with explicitly triggered control, can demonstrate proportion congruency effects--which are based on implicit learning of task regularities and thus indicative of implicitly triggered control. In a series of five experiments, preschoolers indeed demonstrated significant proportion congruency effects, including both list-wide proportion congruency (LWPC) and item-specific proportion congruency (ISPC) effects, in a Stroop-like task and in a flanker task. These effects did not increase with age, contrary to what is typically observed for explicit control. These results demonstrate that young children show early evidence of cognitive control--including proactive control--when it is triggered by implicit events, at an age where explicit control, and particularly proactive control, is not yet functional. By showing evidence of an early ability for fine-grained adjustments of cognitive control when control cues are learned implicitly, these results support the proposed functional dissociation between explicit and implicit cognitive control.