The ability to detect and monitor errors enables us to maintain optimal performance across tasks. One neurophysiological index of error monitoring is the error-related negativity (ERN), a fronto-central negative deflection peaking between 0-150 ms following an erroneous response. The developmental literature has illustrated age-related differences in the ERN and its association with anxiety during development. However, this literature focuses on the between-person differences of the ERN and cannot speak to whether there exist meaningful within-person changes in the ERN throughout a task. We examined the within-person variations of the ERN in 115 community-dwelling nine-to-12-year-olds (66 girls; Mean age/SD=11.00/1.16 years), a critical period for the development of error processing and anxiety psychopathology. Participants completed an EEG Go/No-Go task and reported their anxiety symptoms. Multilevel growth analyses yielded significant within-person, curvilinear changes in the ERN throughout the task. Youths’ trial-level ERN increased (i.e., became more negative) as more errors were made earlier in the task, but decreased as subsequent errors were committed later in the task. This curvilinear pattern was evident in older, but not younger, youths. Age further interacted with anxiety symptoms: younger youths with higher anxiety showed a continuous increase in the ERN throughout the task, whereas older youths with higher anxiety showed an increase in the ERN with early errors, followed by a decline in the ERN in later error trials. Our study contributed novel evidence for the development of the ERN and the underlying mechanisms of the ERN-anxiety relationship that cannot be captured by between-person approaches.