Research Findings: The present study aimed to investigate whether the trial-to-trial intraindividual reaction time variability (IIV), which reflects prefrontal cortex activity related to attentional and cognitive control, mediated the longitudinal effects of earlier harsh discipline on children's later externalizing problems. The study involved 235 elementary school children (M[subscript age] = 7.25, 46% female) who underwent up to three repeated assessments. Specifically, the Chinese version of the Parent-Child Conflict Tactics Scale and the Child Behavior Checklist were completed by the children's mothers to assess harsh discipline (including corporal punishment and psychological aggression) and the children's externalizing problems at three assessment waves. In addition, the children's IIV was measured using the Go/No-Go task at three assessment waves. The results from structural equation modeling revealed that the longitudinal path between corporal punishment at T1 and externalizing problems at T3 was partially mediated by IIV at T2. This suggested that more corporal punishment children experienced, the greater IIV and the more externalizing problems they will develop at a later stage. Practice or Policy: The findings highlight the proposed process model, which shows that corporal punishment impairs the development of children's prefrontal function as indexed by IIV, ultimately increasing the risk of externalizing problems.