Internalizing and externalizing psychiatric disorders among children are common and debilitating, affecting family interactions, learning and peer relations. The aim of the present quasi-randomised pilot-study was to investigate preliminary effects of a mentalization-based time-limited treatment (MBT-C) for children with mixed psychiatric disorders. The trial comprised 17 children, aged 4–11 with mixed disorders, and their parents, admitted to an outpatient psychotherapy clinic. Quasi-randomization allocated patients to 12 sessions MBT-C with parallel parent support, or wait-list control. Compared to wait-list controls, significant improvements were observed in child pathology (d = 1.23, p =.006), therapist-rated global functioning (d = 1.73, p =.002), parent-perceived overall distress and impairment (d = 1.42, p =.017), and child-perceived emotional distress (d = 1.32, p =.024). No significant effects were observed for parent-perceived symptoms (d = 0.41, p =.28). Within-group changes and long-term effects were calculated for all participants including the wait-list after being crossed-over to treatment. Results were either maintained or further improved at 6- and 12-months follow-ups. This trial provides preliminary support for MBT-C in children with mixed disorders.