Individuals diagnosed with a depressive disorder have been found to show reduced reactions to emotional information consistent with the hypothesis of an emotional context insensitivity. However, there are contradictory findings of enhanced reactivity and mood-congruent processing. Electroencephalography (EEG) recordings of the late positive potential (LPP) can display such blunted or enhanced activity. Due to these contradictory findings, there is a need to clarify the role of the LPP in the emergence and presence of depressive disorders especially in children. We used an emotional Go/NoGo task to investigate modulations of the LPP to emotional (fearful, happy, sad) and calm faces in a sample of children and adolescents (age 11;00-14;11) diagnosed with a depressive disorder according to diagnostic parent interviews (K-SADS-PL) (n = 26) compared to a group of age-matched healthy controls (n = 26). LPP positivity was attenuated in children and adolescents with a depressive disorder as well as with higher self-reported depressive symptoms, suggesting reduced reactivity to emotional and calm faces. This is the first study to find generally blunted LPP responses in a clinical sample of depressed youth across reporters. Such dysfunctional modulation of neural activity may represent a potential biomarker for depressive disorders. The results call for further prospective studies investigating the course of the LPP before and after the onset of a depressive disorder in youth.