This study examined the impact of a school readiness intervention on external response monitoring in children in foster care. Behavioral and event-related potential (ERP) data were collected during a flanker task from children who received the Kids In Transition to School (KITS) Program (n = 26) and children who received services as usual (n = 19) before and after the intervention. While there were no significant group differences on the behavioral data, the ERP data for the two groups of children significantly differed. Specifically, in contrast to the children who received services as usual, the children who received the KITS Program displayed greater amplitude differences between positive and negative performance feedback over time for the N1, which reflects early attention processes, and feedback-related negativity, which reflects evaluation processes. In addition, although the two groups did not differ on amplitude differences between positive and negative performance feedback for these ERP components before the intervention, the children who received the KITS Program displayed greater amplitude differences than the children who received services as usual after the intervention. These results suggest that the KITS Program had an effect on responsivity to external performance feedback, which may be beneficial during the transition into kindergarten.