An increasing number of children are conceived using assisted reproduction technologies (ART), but little is known about the long-term risk in terms of mental health outcomes. All twin and singleton children conceived via ART and born in 1995 were sampled from the Danish in vitro fertilization registry and matched to four spontaneously conceived (SC) children. The children were followed-up at the age of 3, 7, 14 and 18 years in the Danish Psychiatric Central Research Registry, to estimate the prevalence of all possible psychiatric diagnoses; dimensional aspects of psychopathology were assessed at the age of 14 years, using the Child Behavior Checklist (CBCL). The study included 858 ART children and 3436 SC children. ART and SC children were not clinically distinguishable on either the categorical measures of psychopathology at the age of 3, 7, 14 and 18 years, or on CBCL scale scores at the age of 14 years. The findings remained nonsignificant after controlling for sample differences. This large, prospective, nationwide cohort study provides evidence to support that ART exposure does not increase the risk of adverse mental health outcomes considered as a whole, while power was limited to discard an effect of ART on specific mental health disorders.