Abstract Objective To document longitudinal changes in internalizing and externalizing behavioral symptoms after mild traumatic brain injury (mTBI) sustained in early childhood (i.e., between 18 and 60 months of age). Methods Participants (N = 226) were recruited to one of three groups: children with mTBI, typically developing children and orthopedic injured children. The Child Behavior Checklist was used to document the presence of internalizing and externalizing behaviors at 6, 18, and 30 months postinjury. Linear mixed-model analyses were used to examine group effects on the trajectory of internalizing and externalizing behavioral manifestations over 30 months postinjury. Results Children who sustain mTBI during the preschool period have higher rates of internalizing and externalizing behavioral symptoms at the initial assessment time point and these symptoms persist up to 30 months postinjury. Moreover, results indicate that for up to 18 months postinjury, significantly more children with mTBI present behavioral difficulties that may require some form of clinical attention (i.e., scores in the borderline or clinical range), than do their orthopedically injured and noninjured peers. Conclusions Sustaining mTBI early in life may lead to long-lasting behavioral changes in young children (i.e., at least 30 months). These changes are likely the product of a complex interplay between neurological and non-neurological factors, both contributing to generating and maintaining behavioral difficulties.