Social connectedness (SC), as a sense of belonging and a psychological bond a person may feel towards other people or groups, is imperative for the positive mental and physical development of children and early adolescents. Particularly children and early adolescents with a mental disorder often face difficulties feeling socially connected and experience the detrimental effects of loneliness. The present systematic review aims to investigate how far SC differs in children and early adolescents with a mental disorder compared to in those that develop neurotypically. Furthermore, it aims to examine the determinants of SC and predominant SC measurement techniques applied in youth with a mental disorder. Following a systematic PRISMA approach, 33 studies were included. In the majority of studies, SC was reduced in the affected population, with varying manifestations over different diagnoses. Determinants could be divided into skills, behavioral and social aspects, and symptoms. Various measurement techniques were applied, exploring friendship quality, loneliness, and peer relations along several dimensions. Interventions and possibilities of influencing SC in certain disorders seems possible and necessary to bring SC more into the focus of daily clinical routine and prevent adverse outcomes in this vulnerable population.