Mild traumatic brain injury (mTBI) is a growing public health concern, and it is a common injury amongst children and adolescents. Existing evidence suggests that mTBI in youth may be related to both externalizing and internalizing symptoms. However, many existing studies fail to control for pre-injury symptoms or consider the potential interaction between mTBI and pre-injury symptom levels. The current study employed data from a longitudinal sample (N = 1,803) of youth from the Project on Human Development in Chicago Neighborhoods to assess the interaction between pre-injury externalizing and internalizing symptoms and outcomes following mTBI in youth. The results showed, contrary to our expectations, that participants with lower pre-injury symptoms were at a greater risk for increased psychopathology following mTBI, compared to participants with higher pre-injury symptoms and non-injured participants. Potential explanations for the results and implications are discussed.