Quantitative EEG (qEEG) has emerged as a potential intermediate biomarker for diagnostic clarification in mental illness. This systematic review examines published studies that used qEEG in youth with psychiatric illness between 1996 and 2017. We conducted a comprehensive database search of CINAHL, PubMed, and Cochrane using the following keywords: "quantitative EEG" and depression (MDD), anxiety, attention-deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), eating disorder, conduct, substance use, schizophrenia, post-traumatic stress disorder, and panic disorder. Our search yielded 516 titles; 33 met final inclusion criteria, producing a total of 2268 youth aged 4-18. qEEG was most frequently studied as a potential diagnostic tool in pediatric mental illness; few studies assessed treatment response. Studies show higher theta/beta ratio in ADHD vs healthy controls (HC). The most consistent finding in ASD was decreased coherence in ASD vs HC. Studies show MDD has lower temporal coherence and interhemispheric coherence in sleep EEGs than HC. Further research is needed in the areas of mood, anxiety, ASD, and relationship to treatment. It remains unknown if abnormalities in qEEG are nonspecific markers of pediatric psychiatric illness or if they have the potential to differentiate types of psychopathology.